The document discusses age-related changes in visual function based on a lecture. It covers 3 main topics:
1) Optical changes like increased light absorption and scatter in the lens and other structures, reducing retinal illumination.
2) Neural changes in the retina like loss of rods, cones and optic nerve fibers. These neural changes may underlie declines in visual functions.
3) Specific age-related changes in visual functions including reduced acuity, contrast sensitivity, dark adaptation, color vision and oculomotor abilities. Both optical and neural factors contribute to these declines.
Poster summarizing undergraduate thesis research with the Russell and Menon groups at the UMass Amherst Materials Research Science and Engineering Center.
EQUOS Enterprise Mobility Services Mobile Web 2.0 | www.equos.bizPrabha Shankar
EQUOS provide mobile services for brands, small & medium business enterprises, media & advertising agencies and mobile operators in the Mobile 2.0 and Web2.0 domains.
Customers channelize EQUOS value-added services (VAS) to mobile consumers via their mobiles, tablets and internet devices in an ‘always-on’, location-based context on a 24x7, 365-day basis.
EQUOS Technology | www.equos.biz
Poster summarizing undergraduate thesis research with the Russell and Menon groups at the UMass Amherst Materials Research Science and Engineering Center.
EQUOS Enterprise Mobility Services Mobile Web 2.0 | www.equos.bizPrabha Shankar
EQUOS provide mobile services for brands, small & medium business enterprises, media & advertising agencies and mobile operators in the Mobile 2.0 and Web2.0 domains.
Customers channelize EQUOS value-added services (VAS) to mobile consumers via their mobiles, tablets and internet devices in an ‘always-on’, location-based context on a 24x7, 365-day basis.
EQUOS Technology | www.equos.biz
The most important decision a manager makes every day is who allows in the door to help him take care of his customer. The presentation focuses on the keys to hire the right people so you can build a front-line workforce that will help you build your bottom line.
Realty Executives Midwest has joined with Selfless Opportunities with the Go Generous Program. We invite all of our Executives and friends to go to our site to link to our Community Events Channel and post your event for free. We are grateful for the opportunity to make a difference, and we invite you to do the same.
The most important decision a manager makes every day is who allows in the door to help him take care of his customer. The presentation focuses on the keys to hire the right people so you can build a front-line workforce that will help you build your bottom line.
Realty Executives Midwest has joined with Selfless Opportunities with the Go Generous Program. We invite all of our Executives and friends to go to our site to link to our Community Events Channel and post your event for free. We are grateful for the opportunity to make a difference, and we invite you to do the same.
Asthenopia: A Technology Induced Visual ImpairmentDominick Maino
Vision systems evolve over generations based on the needs of the users and the environment. In humans, evolutionary pressures led to the development of the need for clear distance visual acuity and binocular three-dimensional (3D) stereoscopic vision. These visual skills enabled us to effectively respond to threats in the environment that were distant and constantly
changing, and improved our odds of being the hunter rather than the hunted. When Johannes
Gutenberg developed the process for modern book printing in the mid 15th century, he set in motion the shift in visual demands away from the importance of seeing clearly at distance and toward
a time intensive two-dimensional near-point task such as reading....read more...
Optometric examination and management of geriatric problems.pptxAnisha Heka
Normal age related changes
Common pathological changes with age
Optometric examination of geriatric population
Complications in examination of older patient
Vision Corrections in older patient
These simplified slides by Dr. Sidra Arshad present an overview of the non-respiratory functions of the respiratory tract.
Learning objectives:
1. Enlist the non-respiratory functions of the respiratory tract
2. Briefly explain how these functions are carried out
3. Discuss the significance of dead space
4. Differentiate between minute ventilation and alveolar ventilation
5. Describe the cough and sneeze reflexes
Study Resources:
1. Chapter 39, Guyton and Hall Textbook of Medical Physiology, 14th edition
2. Chapter 34, Ganong’s Review of Medical Physiology, 26th edition
3. Chapter 17, Human Physiology by Lauralee Sherwood, 9th edition
4. Non-respiratory functions of the lungs https://academic.oup.com/bjaed/article/13/3/98/278874
New Drug Discovery and Development .....NEHA GUPTA
The "New Drug Discovery and Development" process involves the identification, design, testing, and manufacturing of novel pharmaceutical compounds with the aim of introducing new and improved treatments for various medical conditions. This comprehensive endeavor encompasses various stages, including target identification, preclinical studies, clinical trials, regulatory approval, and post-market surveillance. It involves multidisciplinary collaboration among scientists, researchers, clinicians, regulatory experts, and pharmaceutical companies to bring innovative therapies to market and address unmet medical needs.
micro teaching on communication m.sc nursing.pdfAnurag Sharma
Microteaching is a unique model of practice teaching. It is a viable instrument for the. desired change in the teaching behavior or the behavior potential which, in specified types of real. classroom situations, tends to facilitate the achievement of specified types of objectives.
Local Advanced Lung Cancer: Artificial Intelligence, Synergetics, Complex Sys...Oleg Kshivets
Overall life span (LS) was 1671.7±1721.6 days and cumulative 5YS reached 62.4%, 10 years – 50.4%, 20 years – 44.6%. 94 LCP lived more than 5 years without cancer (LS=2958.6±1723.6 days), 22 – more than 10 years (LS=5571±1841.8 days). 67 LCP died because of LC (LS=471.9±344 days). AT significantly improved 5YS (68% vs. 53.7%) (P=0.028 by log-rank test). Cox modeling displayed that 5YS of LCP significantly depended on: N0-N12, T3-4, blood cell circuit, cell ratio factors (ratio between cancer cells-CC and blood cells subpopulations), LC cell dynamics, recalcification time, heparin tolerance, prothrombin index, protein, AT, procedure type (P=0.000-0.031). Neural networks, genetic algorithm selection and bootstrap simulation revealed relationships between 5YS and N0-12 (rank=1), thrombocytes/CC (rank=2), segmented neutrophils/CC (3), eosinophils/CC (4), erythrocytes/CC (5), healthy cells/CC (6), lymphocytes/CC (7), stick neutrophils/CC (8), leucocytes/CC (9), monocytes/CC (10). Correct prediction of 5YS was 100% by neural networks computing (error=0.000; area under ROC curve=1.0).
Title: Sense of Smell
Presenter: Dr. Faiza, Assistant Professor of Physiology
Qualifications:
MBBS (Best Graduate, AIMC Lahore)
FCPS Physiology
ICMT, CHPE, DHPE (STMU)
MPH (GC University, Faisalabad)
MBA (Virtual University of Pakistan)
Learning Objectives:
Describe the primary categories of smells and the concept of odor blindness.
Explain the structure and location of the olfactory membrane and mucosa, including the types and roles of cells involved in olfaction.
Describe the pathway and mechanisms of olfactory signal transmission from the olfactory receptors to the brain.
Illustrate the biochemical cascade triggered by odorant binding to olfactory receptors, including the role of G-proteins and second messengers in generating an action potential.
Identify different types of olfactory disorders such as anosmia, hyposmia, hyperosmia, and dysosmia, including their potential causes.
Key Topics:
Olfactory Genes:
3% of the human genome accounts for olfactory genes.
400 genes for odorant receptors.
Olfactory Membrane:
Located in the superior part of the nasal cavity.
Medially: Folds downward along the superior septum.
Laterally: Folds over the superior turbinate and upper surface of the middle turbinate.
Total surface area: 5-10 square centimeters.
Olfactory Mucosa:
Olfactory Cells: Bipolar nerve cells derived from the CNS (100 million), with 4-25 olfactory cilia per cell.
Sustentacular Cells: Produce mucus and maintain ionic and molecular environment.
Basal Cells: Replace worn-out olfactory cells with an average lifespan of 1-2 months.
Bowman’s Gland: Secretes mucus.
Stimulation of Olfactory Cells:
Odorant dissolves in mucus and attaches to receptors on olfactory cilia.
Involves a cascade effect through G-proteins and second messengers, leading to depolarization and action potential generation in the olfactory nerve.
Quality of a Good Odorant:
Small (3-20 Carbon atoms), volatile, water-soluble, and lipid-soluble.
Facilitated by odorant-binding proteins in mucus.
Membrane Potential and Action Potential:
Resting membrane potential: -55mV.
Action potential frequency in the olfactory nerve increases with odorant strength.
Adaptation Towards the Sense of Smell:
Rapid adaptation within the first second, with further slow adaptation.
Psychological adaptation greater than receptor adaptation, involving feedback inhibition from the central nervous system.
Primary Sensations of Smell:
Camphoraceous, Musky, Floral, Pepperminty, Ethereal, Pungent, Putrid.
Odor Detection Threshold:
Examples: Hydrogen sulfide (0.0005 ppm), Methyl-mercaptan (0.002 ppm).
Some toxic substances are odorless at lethal concentrations.
Characteristics of Smell:
Odor blindness for single substances due to lack of appropriate receptor protein.
Behavioral and emotional influences of smell.
Transmission of Olfactory Signals:
From olfactory cells to glomeruli in the olfactory bulb, involving lateral inhibition.
Primitive, less old, and new olfactory systems with different path
Recomendações da OMS sobre cuidados maternos e neonatais para uma experiência pós-natal positiva.
Em consonância com os ODS – Objetivos do Desenvolvimento Sustentável e a Estratégia Global para a Saúde das Mulheres, Crianças e Adolescentes, e aplicando uma abordagem baseada nos direitos humanos, os esforços de cuidados pós-natais devem expandir-se para além da cobertura e da simples sobrevivência, de modo a incluir cuidados de qualidade.
Estas diretrizes visam melhorar a qualidade dos cuidados pós-natais essenciais e de rotina prestados às mulheres e aos recém-nascidos, com o objetivo final de melhorar a saúde e o bem-estar materno e neonatal.
Uma “experiência pós-natal positiva” é um resultado importante para todas as mulheres que dão à luz e para os seus recém-nascidos, estabelecendo as bases para a melhoria da saúde e do bem-estar a curto e longo prazo. Uma experiência pós-natal positiva é definida como aquela em que as mulheres, pessoas que gestam, os recém-nascidos, os casais, os pais, os cuidadores e as famílias recebem informação consistente, garantia e apoio de profissionais de saúde motivados; e onde um sistema de saúde flexível e com recursos reconheça as necessidades das mulheres e dos bebês e respeite o seu contexto cultural.
Estas diretrizes consolidadas apresentam algumas recomendações novas e já bem fundamentadas sobre cuidados pós-natais de rotina para mulheres e neonatos que recebem cuidados no pós-parto em unidades de saúde ou na comunidade, independentemente dos recursos disponíveis.
É fornecido um conjunto abrangente de recomendações para cuidados durante o período puerperal, com ênfase nos cuidados essenciais que todas as mulheres e recém-nascidos devem receber, e com a devida atenção à qualidade dos cuidados; isto é, a entrega e a experiência do cuidado recebido. Estas diretrizes atualizam e ampliam as recomendações da OMS de 2014 sobre cuidados pós-natais da mãe e do recém-nascido e complementam as atuais diretrizes da OMS sobre a gestão de complicações pós-natais.
O estabelecimento da amamentação e o manejo das principais intercorrências é contemplada.
Recomendamos muito.
Vamos discutir essas recomendações no nosso curso de pós-graduação em Aleitamento no Instituto Ciclos.
Esta publicação só está disponível em inglês até o momento.
Prof. Marcus Renato de Carvalho
www.agostodourado.com
Flu Vaccine Alert in Bangalore Karnatakaaddon Scans
As flu season approaches, health officials in Bangalore, Karnataka, are urging residents to get their flu vaccinations. The seasonal flu, while common, can lead to severe health complications, particularly for vulnerable populations such as young children, the elderly, and those with underlying health conditions.
Dr. Vidisha Kumari, a leading epidemiologist in Bangalore, emphasizes the importance of getting vaccinated. "The flu vaccine is our best defense against the influenza virus. It not only protects individuals but also helps prevent the spread of the virus in our communities," he says.
This year, the flu season is expected to coincide with a potential increase in other respiratory illnesses. The Karnataka Health Department has launched an awareness campaign highlighting the significance of flu vaccinations. They have set up multiple vaccination centers across Bangalore, making it convenient for residents to receive their shots.
To encourage widespread vaccination, the government is also collaborating with local schools, workplaces, and community centers to facilitate vaccination drives. Special attention is being given to ensuring that the vaccine is accessible to all, including marginalized communities who may have limited access to healthcare.
Residents are reminded that the flu vaccine is safe and effective. Common side effects are mild and may include soreness at the injection site, mild fever, or muscle aches. These side effects are generally short-lived and far less severe than the flu itself.
Healthcare providers are also stressing the importance of continuing COVID-19 precautions. Wearing masks, practicing good hand hygiene, and maintaining social distancing are still crucial, especially in crowded places.
Protect yourself and your loved ones by getting vaccinated. Together, we can help keep Bangalore healthy and safe this flu season. For more information on vaccination centers and schedules, residents can visit the Karnataka Health Department’s official website or follow their social media pages.
Stay informed, stay safe, and get your flu shot today!
Basavarajeeyam is a Sreshta Sangraha grantha (Compiled book ), written by Neelkanta kotturu Basavaraja Virachita. It contains 25 Prakaranas, First 24 Chapters related to Rogas& 25th to Rasadravyas.
Adv. biopharm. APPLICATION OF PHARMACOKINETICS : TARGETED DRUG DELIVERY SYSTEMSAkankshaAshtankar
MIP 201T & MPH 202T
ADVANCED BIOPHARMACEUTICS & PHARMACOKINETICS : UNIT 5
APPLICATION OF PHARMACOKINETICS : TARGETED DRUG DELIVERY SYSTEMS By - AKANKSHA ASHTANKAR
Adv. biopharm. APPLICATION OF PHARMACOKINETICS : TARGETED DRUG DELIVERY SYSTEMS
Physiology of Aging3
1. Week 11 The Physiological Impact of Ageing: Part III Changes in Visual Function
Introduction
Many elderly people, despite being classified as free from ocular disease, often experience visual
problems that interfere with their daily lives. With respect to visual function the dividing line between
what are considered as normal age changes and those due to pathology is fairly arbitrary. This lecture
will concentrate on those aspects of visual function that deteriorate with age in the absence of
pathology. Nevertheless it is important to point out that ocular disease and vision loss are more
prevalent in the elderly population the four leading causes of visual impairment being: age-related
macular degeneration, cataract, glaucoma and diabetic retinopathy.
There are several reasons why certain aspects of visual function deteriorate with increasing age. The
radiation reaching the eye is significantly altered by changes in transmission characteristics of the iris,
the crystalline lens and to a smaller extent the cornea. The retina itself undergoes neural changes that
are becoming better understood. The extent to which changes in the visual pathway and central nervous
system is still debated but at least some of the neurones involved in visual processing are lost with age.
Optical Changes
Vision is ultimately dependent upon the ability of the optics of the eye to produce a good image on the
retina. Age-related changes in the optics of the eye can therefore have a major effect on the visual
function of the elderly patient.
One major change is an increase in the amount of light that is absorbed by the ocular
structures along the visual axis. This reduction in the transmission of light and loss of transparency is
greatest in the crystalline lens but also occurs to a lesser extent in the cornea and vitreous and results in
a decrease in the amount of light that reaches the retina, i.e. reduces retinal illumination. The increase
in absorption is much greater for shorter wavelength light (see fig. 7 of lecture for Wk 10).
Transmission/Absorption
If Ii is the intensity of monochromatic light
incident upon an ocular structure and It is
the intensity of light which passes through
it, then It/Ii = the transmission, T. The
optical density D, of a structure is given as:
D = -log T
The higher the value of T the more light
that passes through the structure. That light
which is not transmitted must be absorbed.
Absorption and transmission are related as
Figure 1 Transmission of light by ocular structures
Low transmission in the short wavelengths is
due largely to the lens and is reduced even
further in the elderly eye. The dip in the blue
region of the spectrum (440-500 nm) is due
to absorption by the yellow macular pigment.
This macular pigment differs widely in
concentration between different individuals
but does not differ significantly with age.
Figure 2 Resultant transmission by ocular media
2. Another major change in the optics of the eye in later life is increased intra-ocular light scatter which
although may not lead to a reduction in the amount of retinal illumination, does cause image
degradation due to loss of image contrast via the introduction of veiling luminance.
Neural Changes
Over the past decade there have been increasing numbers of reports that suggest that age-related
changes do occur in the neural components of the retina and the rest of the visual pathway. We are still
unclear as to precisely what influence these neural changes have on visual function in the elderly but
some of the changes that occur with age are listed below:
• loss of rods and cones
• ↓ in cone density (in rhesus monkeys)
• ↓ human cone photopigment density
• structural changes in the outer segments of photoreceptors
• ↓ in the number of optic nerve fibres
Age-related Changes in Visual Function
Acuity
The measurement of visual acuity is the standard clinical method of evaluating pattern vision. Acuity is
typically assessed using high contrast, high luminance letter charts. There have been many studies that
have investigated how acuity changes with age the results of which are summarised in figure 3 below:
Figure 3. Mean letter acuity
as a function of age as
reported in several studies.
All studies appear to agree that acuity decreases with increasing age, but there are discrepancies
between the studies in terms of the rate of decline, the age of onset of the decline and so forth. These
differences are likely to be due in the main to methodological inconsistencies between the studies.
Weale (1975) has argued that a large proportion of this acuity loss is due to neural deterioration and
cell death in the visual pathway (see below for more on optical vs neural theories).
Contrast Sensitivity
Spatial
Unlike measures of vision such as Snellen acuity, spatial contrast sensitivity testing examines pattern
vision at low as well as high contrast for a range of stimulus sizes (or more correctly spatial
frequencies). When spatial contrast sensitivity is measured for elderly subjects and compared to the
performance in the younger population it is found that older subjects exhibit a loss in sensitivity at
higher spatial frequencies (see figure 4). The magnitude of this loss increases with increasing spatial
frequency and also when the surrounding light levels decrease.
3. 1000
71 yrs
21 yrs
Contrast Sensitivity
100
10
Figure 4. Contrast sensitivity as
a function of spatial frequency.
1
0.1 1 10 100
Spatial Frequency (cycles/deg)
Temporal
Temporal contrast sensitivity measurement assesses the sensitivity of the visual system to stimuli
which change as a function of time (e.g. flickering or moving). Studies have shown that there is a loss
in temporal resolution for luminance modulated uniform fields. With drifting sinusoidal grating stimuli
there is a loss of contrast sensitivity with ageing even at low temporal frequencies, for both colour and
luminance stimuli (see figure 5). Motion sensitivity also decreases with age.
Figure 5. Temporal contrast sensitivity
(motion direction discrimination) for
colour and luminance stimuli in young
and old subjects
Optical vs Neural Theories of Sensitivity Loss
Although the underlying basis of this loss in sensitivity with age has not be fully explained it would
seem that it is not due to refractive error, senile miosis of the pupil, increased light absorption or
cognitive factors associated with decision making or setting a threshold. This conclusion has been
reached following studies on elderly subjects with intra-ocular lens implants and using laser stimuli
which by-pass the eye’s optics yet still indicate a loss of sensitivity at high spatial frequencies. It would
appear that neural changes that occur in the retina and visual system are the most likely cause of
contrast sensitivity loss with age.
Various studies on ageing have tried to answer the question whether there is a selective
deterioration of one or other of the two main neural processing pathways, the Parvo- (P) or Magno-
(M) cellular pathways (see Spear, 1993). Given the roles played by the two pathways in the visual
systems of non-human primates, a deterioration in the high spatial and low temporal frequency domain
would suggest a selective P system deficit, whilst the loss of motion sensitivity would suggest a decline
in the activity of the M system. The non-specific losses in contrast sensitivity for both colour and
luminance patterns (Fiorentini et al., 1996), suggest that ageing affects both P & M systems in
generalised manner.
4. Colour Vision
With regards to the effects of ageing on colour vision there seems to be little doubt that there is a
reduction in sensitivity of the short wavelength sensitive (S) cones in later life. Whether this loss
extends to the middle (M) and long (L) wavelength sensitive cones is still the matter of debate. The loss
of S-cone sensitivity may be due in part to optical factors since we know that absorption for short
wavelength sensitive light increases in the elderly crystalline lens. But even when this increased
absorption is accounted for S-cone sensitivity is still reduced in the older eye, suggesting that neural
age changes must also play some part. Some researchers argue that the S-cone system is more
susceptible to damage by the ageing process than either the L or M-cone systems.
As a consequence of this reduced S-cone sensitivity elderly patients tend to exhibit more tritan like
performance in colour vision tasks. This was demonstrated by Knoblauch et al. (1987) who used the
Farnsworth-Munsell 100 hue test to assess the variations of colour vision with age as well as
luminance. They showed increases in the error score with age with scores reaching a maximum along
the near vertical axis (see figures 6 & 7) indicating tritanopia.
Fig 7
Fig 6
Other changes that occur in colour vision include changes in the isoluminant point. When two colours
are alternated at a fast rate (15Hz) the relative luminances of the colours can be adjusted until perceived
flicker is minimum. At the minimum flicker point the two colours are equi- or iso-luminant. This
technique is known as heterochromatic flicker photometry (HFP). When older subjects perform this
task they tend to require higher levels of green in order to achieve minimum flicker.
FIGURE 8. (A) Contrast sensitivity as
a function of colour ratio a horizontal
grating 1 c/deg sinusoidally reversed
in contrast at 15Hz. The colour ratio
0.47 at which contrast sensitivity was
minimum was taken as the
equiluminant value.
(B) Equihrminant colour ratios of the
ten young (o) and ten old subjects (•)
participating in the psychophysical
experiment, as a function of age.
Arrows indicate the means for the 2
age groups.
Green iso- Red
From Fiorentini et al. (1996)
luminant
5. Dark Adaptation
Dark adaptation is the time dependent increase in visual sensitivity that occurs in darkness following
exposure to bright illumination levels and reveals fundamental information about the function of rods
and cones.
Numerous studies have shown that the elderly have elevated thresholds (i.e. decreased
sensitivity) throughout the entire time course of dark adaptation in both the rod and cone portions of the
function (see figure 9). The mechanisms that underlie these changes in adaptation in the elderly may be
both neural and optical in origin.
Threshold
Time (mins)
Figure 9 Dark adaptation for different age groups
Oculomotor Function
Age-related changes in dynamic oculomotor control have been reported.
• during saccadic eye movements older subjects show an increased latency of saccadic
onset. To a lesser extent saccade duration and velocity may decrease with age.
• older people also exhibit significantly slower smooth pursuit eye movements for
targets moving at speeds greater than 10º/s.
• a decrease in the ability to resolve the detail of moving stimuli (Dynamic VA)
Binocular Vision
There is some indication that binocular summation and stereoacuity decrease with age. However, the
conclusions drawn from studies of binocular vision must be treated with some caution since few of the
studies carried so far have 1) been rigorous about the ocular health and refractive correction of the
participants, 2) yielded direct measures of stereoacuity or 3) used stereo tests that adequately isolated
the cue of binocular disparity.
Visual Fields
Visual sensitivity across the whole visual field is adverse affected by the processes of ageing. Studies
have indicated isopter constriction in older adults as well as a generalised loss in sensitivity throughout
the whole of the visual field (see figure 10).
Fig. 10. Foveal threshold to a
small light stimulus as a
function of age
6. Another approach to examining the visual field is to assess the ‘functional’ or ‘useful’ field of view
which involves the localisation and identification of complex stimuli in the periphery. The limits of
useful field of view are affected by many factors such as the presence secondary tasks and background
distractor stimuli. The impact of these variables is much greater for older people.
Visual Electrophysiology
The visual evoked potential (VEP) is a composite electrical signal generated by the occipital cortex in
response to a visual stimulus. Generally the amplitudes of these responses decrease as a function of age
and their latency increases. This is found across a wide range of stimuli that are used to elicit the
responses (see figure 11).
Fig 11 Mean latencies of the VEPS of young
(open columns) and old (hatched columns)
subjects
Researchers argue that age-related increases in VEP latency are neurally based and are related to
decreases in nerve conduction velocities caused by demyelination and retinal deterioration.
Objectives:
Following this lecture the student should be able to:
1. Describe the major changes that occur in visual function as a result of ageing.
2. Discuss the differential roles of optical and neural factors as reasons for decline in visual
function.
Further Reading.
Weale RA. (1992) The Senescence of Human Vision. Oxford Medical Publications. (Chapters 3 & 4)
Library S612.84
Owsley C. & Sloane ME. (1990). Vision and Aging. In: Handbook of Neuropsychology Vol 4. Eds.
Boller F. & Grafman J. pp 229-249. Elsevier Science Publishers.
Knoblauch, K et al. (1987). Age and illuminance effects in the Farnsworth-Munsell 100 hue test.
Applied Optometry 26, 1441-1448.
Spear,P. D. (1993). Neural bases of visual deficits during aging. Vision Research, 33, 2589-2609.
Fiorentini, A. et al (1996). Visual Ageing: unspecific decline of the responses to luminance and colour.
Vision Research, 36, 3557- 3566.