SENSATION & PERCEPTION

Sensation: process of detecting the presence of a stimulus

Perception: process of interpreting the stimulus

Transduction: the process through which sensation is converted into sensory neural impulses

Thalamus: the gateway to the cortex. All senses except olfaction go through the thalamus.

VISION

• The eye: visual input (light) shines through cornea and lens, which refracts light to form

a focused, inverted image on the fovea of the retina.

• Fovea: center of retina. Only at fovea we form a sharp, clear image of the object

Receptor cells

• Rods: distributed across the retina except for the fovea. Responsible for low-resolution

vision, such as peripheral vision or vision at night.

• Cones: mainly located in the foveal area, much less on the rest of the retina as the

distance from the fovea increases. Responsible for acute and sharp vision, also our vision during

the day when lighting is ample.

• Transduction on the retina: Light first reaches the deep layer of the retina, gets processed by the photoreceptors

(rods and cones) embedded there, which convert them to neural signals in the form of action potentials.

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Visual pathway

• Visual field is divided into left and right hemifields (LVF, RVF) relative to the

fixation point (+).

• Most input from each visual field is projected to the primary visual cortex in the

contralateral hemisphere after the fibers synapse in the lateral geniculate nucleus

(LGN) in the thalamus. A small percentage of visual fibers of the optic nerve

terminate in subcortical structures.

Primary visual cortex/V1/Striate cortex: in the occipital lobe

• Extracts basic information from the visual scene (e.g. edges, orientations,

wavelength of light)

Extrastriate cortex (Secondary & Tertiary visual cortex):

• Receive inputs from V1

• V4: color.

• V5/MT area: sensitive to the direction and speed of moving object.

Visual system organization

• Visual system is divided into specialized areas maintain maps for different aspects of visual information

• Information from these areas is integrated to give the final representation of stimuli.

DEFICITS IN VISUAL PERCEPTION

Deficits due to damage to V1

• Hemianopia: lesion to V1 in one hemisphere such that loss of vision is r estricted to one visual

field.

• Scotoma: blindness to discrete areas of the visual field due to small lesions to V1

Deficits of color vision

• Achromatopsia

o Cause: acquired (cortical damage to V4) or genetic (malfunction of the retinal

phototransduction pathway)

o Color perception is lost, but can distinguish objects by luminance (shades of gray)

Deficits in perceiving motion

• Akinetopsia:

o Cause: lesion to V5

o Unable to perceive motion in a continuous, coherent fashion =>likely to miss motion

cues, thus impaired in determining direction of movement and predicting future location

of moving objects

CORTICAL PATHWAYS

• The “what” pathway: from the occipital lobe to the temporal lobe → ventral

o Recognizing the identity of objects

o Inferior temporal lobe: larger receptive fields compared to V1; complexity of information

increases from posterior to anterior temporal lobe

• The “where” pathway: from the occipital lobe to the parietal and frontal lobe → dorsal

o Recognizing the location of objects (for action involving them)

FAILURES IN OBJECT RECOGNITION

Agnosia: inability to recognize visually presented objects, but recognition can occur through

other modalities (e.g., touch, audition). Impairment can be at the level of perceptual integration

or at the level of the link between visual perception and memory. It’s not due to memory loss,

sensory loss, or motor impairment.

• Apperceptive agnosia: ventral-stream disorder. The patient’s ability to achieve object

constancy is disrupted; recognition is impaired for objects presented

from non-standard views or with limited amount of information.

Problem copying drawings.

• Associative agnosia: Impaired with bilateral posterior (occipital-temporal) lesions. Failure of visual

object recognition that cannot be attributed to perceptual abilities. Impairment related to

semantic categorization

o Impaired ability to understand the meaning of an object => “matching by

function” test

o Can copy drawings, but cannot draw from memory

Prosopagnosia: deficit in face recognition that cannot be attributed to dementia or visual perceptual problems. In

extreme cases may not even recognize their spouses, children, or their own face. But can correctly identify visually

other objects.

- Might compensate for their deficits by relying on a variety of cues

- Acquired and congenital prosopagnosia

- Fusiform face area: face-specific vs. expertise hypothesis