Sensation Biological Psychology Neural Mechanisms

1. VISION
Pavithra Narasimhan

2. The Pupil and the Lens
• The amount of light reaching the retinas is regulated by
the contractile tissue, the irises.
• Light enters the eye through the pupil, the hole in the
iris.
• illumination represents a compromise between
sensitivity (the ability to detect the presence of dimly
lit objects) and acuity (the ability to see the details of
objects).
• Behind each pupil is a lens, which focuses incoming
light on the retina

3. Ctd..
• When we focus objects nearby, the tension on the
ligaments holding each lens in place is adjusted by
the ciliary muscles, and the lens assumes its natural
cylindrical shape.
• When we focus on a distant object, the lens is
flattened.
• Accommodation is the process of adjusting the
configuration of the lenses to bring images into focus
on the retina is called

4. Eye Position

6. Binocular Disparity
The difference in the position of the same image
on the two retinas is greater for close objects
than for distant objects.
Therefore, your visual system can use the
degree of binocular disparity to construct one
three-dimensional perception from two two-
dimensional retinal images

8. The Retina
The retina is composed of five layers of different
types of neurons:
• Receptors
• Horizontal cells
• Bipolar cells
• Amacrine cells
• Retinal ganglion cells.
• Retinal neurons communicate both chemically via
synapses and electrically via gap junctions

9. Translation of Light into Neural Signals
The retina converts light to neural signals, conducts
them toward the CNS.
• It participates in the processing of the signals.
The incoming light is distorted by the retinal
tissue through which it must pass before reaching
the receptors.
• The bundle of retinal ganglion cell axons to leave
the eye, there must be a gap in the receptor
layer; this gap is called the blind spot.

11. Cone and Rod Vision
• Cone-shaped receptors called cones, and rod-
shaped receptors called rods.
• species active only in the day tend to have
cone-only retinas and species active only at
night tend to have rod-only retinas.

13. Cortical Mechanisms of Vision
• The entire occipital cortex as well as large
areas of temporal cortex and parietal cortex
are involved in vision.
• Visual cortex is often considered to be of three
different types
• Primary visual cortex
• secondary visual cortex
• visual association cortex

15. Cortex Ctd..
• The primary visual cortex is located in the
posterior region of the occipital lobes.
• Most areas of secondary visual cortex are
located in two general regions: in the
prestriate cortex and in the inferotemporal
cortex.

16. Damage to Primary Visual Cortex:
Scotomas and Completion
• Damage to an area of the primary visual
cortex produces a scotoma, an area of
blindness.
perimetry test.
• Patients with extensive scotomas are not
consciously aware of their deficits.
• Due completion.

18. Functional Areas of Secondary and
Association Visual Cortex
• Secondary visual cortex and the portions of
association
• cortex that are involved in visual analysis are both
composed of different areas, each specialized for
a particular type of visual analysis.
• The neurons in each functional area respond
most vigorously to different aspects of visual
stimuli (e.g., to their color, movement, or shape)

19. Dorsal and Ventral Streams
• The dorsal stream flows from the primary
visual cortex to the dorsal prestriate cortex to
the posterior parietal cortex.
• ventral stream flows from the primary visual
cortex to the ventral prestriate cortex to the
inferotemporal cortex.

22. Neuropsychological disorders of vision:
• Agnosia is a failure of recognition (gnosis
means to know )
• Prosopagnosia -associated with damage to an
area of the ventral stream
• Akinetopsia associated with damage to an
area of the dorsal stream.

23. Prosopagnosia
• Prosopagnosics can usually recognize a face as a
face,
• but they have problems recognizing whose face it
is. They often report seeing a jumble of individual
facial parts (e.g., eyes, nose, chin, cheeks)
• prosopagnosia results from bilateral damage to
the ventral stream suggests that dorsal-stream
function may be intact

24. Akinetopsia
• Akinetopsia is a deficiency in the ability to see
movement progress in a normal smooth
fashion. Akinetopsia can be triggered by high
doses of certain antidepressants
• Akinetopsia is often associated with damage
to the middle temporal (MT) area of the
cortex. The location of MT near the junction of
the temporal, parietal, and occipital lobes

26. AUDITION

27. Sound Waves

28. Divisions of the Ear
 Outer ear:
 Channel to tympanic
membrane
 Middle ear:
 Ossicles
 Inner ear:
 Cochlea

29. The Cochlea
 The cochlea is formed from three chambers:
 Scala vestibuli and scala media are separated by a membrane
 Scala tympani and scala media are separated by the basilar
membrane
 Hair cells within the organ of Corti transduce sound
waves into nerve impulses
 The organ of Corti consists of
 Basilar membrane (forms the base)
 Tectorial membrane (forms the roof)
 Hair cells in between

30. Auditory Transduction
 Cilia tips are joined by a
fiber link
 Cilia movement produces
tension of the link which
opens an ion channel in
the adjacent tip
 Calcium and potassium
ions flow into the cilia
and produce a
depolarization

31. Auditory Pathways
 Afferent pathways:
 Through cochlear nuclei
 To superior olivary nuclei
 To inferior colliculus
 To medial geniculate
 To auditory cortex
 Efferent pathway:
 Olivocochlear bundle

32. Analysis of the Auditory System
 The various components of the auditory system
detect sounds, determine sound location, and
recognize sound identity
 Lesions placed at different levels of the auditory
system:
 Bilateral auditory cortex: animal can detect pitch,
intensity diff, but not “tunes”
 Brachium of inf. colliculus: animal cannot detect
frequency or intensity differences
 Lateral lemniscus: animal is deaf

33. Somatosenses
 The somatosenses provide information relating to events
on the skin and to events occurring within the body
 The cutaneous senses receive various signals from the skin that
form the sense of touch
 Pressure
 Vibration
 Heating/cooling
 Stimuli that damage tissue (and produce pain)
 Kinesthesia provides information about the body position and
movement
 Kinesthetic signals arise from receptors located within the joints, tendons,
and muscles

34. CUTANEOUS RECEPTORS
• The simplest cutaneous receptors are the free
nerve endings.
• The largest and deepest cutaneous receptors are
the Pacinian corpuscles.
respond to sudden displacements of the skin
but not to constant pressure.
• Merkel s disks and Ruffini endings both adapt
slowly and respond to gradual skin indentation
and skin stretch

36. Morphology of Skin
Dermis
Epidermis

37. Cutaneous Senses
 Three different sensations are reported to the brain
by receptors localized within skin
 Touch involves perception of pressure and vibration of
an object on the skin
Pacinian corpuscles detect deformation of the skin
 Temperature is detected by warmth and cold receptors
Receptor activation is relative to the baseline temperature
The receptors lie at different levels of the skin (cold are
close to the surface of the skin)
 Pain is associated with skin tissue damage

38. Somatosensory Pathways
 The dorsal columns carry information related to touch
(precisely localized)
 The spinothalamic tract carries pain and temperature
signals (poorly localized)
 Somatosensory cortex is organized into columns
 There may be 5-10 cortical maps of the body surface

40. Pain
 Pain serves a functional role for survival
 Persons lacking pain receptors are at great risk
 Pain stimuli induce species-typical escape and
withdrawal responses
 Pain is a motivational force that can activate behavior
 Pain involves tissue destruction induced by
Thermal stimuli
Mechanical force
 Pain reception is poorly localized (as is temperature)
 Pain involves an emotional component (that can be used
to modify the magnitude of pain perception)

41. Pain Receptors
stimulation
 Receptors for pain (nociceptors)
 Free nerve endings networks within the skin that respond to intense
pressure
 Free nerve endings that respond to heat, acids, and capsaicin (the
active ingredient in chili peppers)
 Receptors that are sensitive to ATP
 Pain receptors are found in:
 Skin
 Sheath around muscles, internal organs
 Cornea of the eye
 Pulp of the teeth
 Pain receptors are activated by mechanical, chemical

42. Opiates and Pain
 Exogenous opiates reduce pain reactivity
 Brain produces several endorphins
 Naloxone reverses opiate activity
 Naloxone reversibility is taken as an indication of
opiate involvement
 Focal brain stimulation can reduce pain
 PAG in particular is effective
 Brain stimulation activates a descending pathway
that modulates pain (Basbaum and Fields model)