Perception Midterm

“thinking” about your own perceptual experience

1. your own experience is subjective and thus not verifiable

2. introspection may not reveal underlying processes

a. Color opponency (eg red and green are opposing sensations) b. Lateral inhibition (eg Mach bands)

weakest stimuli we can detect or tell apart

a low sensitivity and vice versa

a. thresholds are limited by noise and thus not all or none

b. detection thus depends on both sensitivity and criterion

c. signal detection theory provides a method for understanding threshold judgments in the presence of noise (undertainty)

Yes

a “filter” that is selective for certain information

an orientation channel will respond only to a narrow range of tilts

if the stimuli influence each other they are encoded by the same channel

adapting to one size affects the threshold only for similar sizes

a measure of the speed of a response

Can be used to infer the type of processing involved in a perception

a. Stroop effect

b. Mental rotation

c. Visual search

studying perceptual deficits in patients with brain damage

fMRI and PET

Electrical potentials

Single unit recording

examining which parts of the brain are active during a perceptual task by monitoring blood flow

examining brain responses to stimuli by measuring the electrical responses on scalp (e.g. visual evoked potential) or surface of the eye (e.g. electroretinogram)

measuring the responses of individual cells by recording their electrical signals

many varieties but 4 basic parts

1. Cell body

2. Dendrites

3. Axon

4. Synapse

contains most of the machinery to keep cell functioning

fibers for receiving information from other neurons

fiber for sending information to other neurons

where neuron makes contact with other neurons

leads to charge differences across the membrane

1. Resting potential

2. Graded potentials

3. Action potential

baseline charge difference when cell is at rest (not stimulated)

passive charge difference in response to stimulation

a. synaptic potential

b. receptor potential

due to influence of other neuron

due to “transduction” of physical stimulus

ex: the absorption of light

brief charge (nerve impulse or “spike”) generated by a neuron to carry information along the axon

Acton potential : large-fixed

Graded potential : small-variable

Action : brief (e.g. 1 msec)

Graded : long

Action : depolarize only

Graded : depolarization or hyperpolarization

Action : axon

Graded : throughout cell

Action : to carry fixed signal over long distances

Graded : to sum together the inputs from other cells

connection between neurons mediated by chemical neurotransmitters

the post-synaptic cell

synaptic connections to the cell

1. Size of response (graded, or number of action potentials)

2. Pattern of responses (e.g. temporal coding of sound frequencies)

3. Pattern of connections (labelled lines)

1. Stimuli are inherently ambiguous

• eg: many different objects could give rise to the same retinal image

2. The brain must therefore make guesses or “unconscious inferences” in order to interpret the image

3. Perception is therefore “indirect” because it requires “information processing”

1. ecological or computational

2. psychological

3. physiological

what is the goal of the perception, and whatproperties of the stimulus or the situation allow the problem to be solved?

Example: optic flow: as we move through the environment the retinal image changes in characteristic ways that provide cues about our motion

what are the processes underlying our perception and what sort of mental representations do they lead to?

how are these processes and representations realized by the nervous system?

1. encoding, representation, and decoding

2. Serial vs parallel processing in sensory systems

3. Multiple-channel models and population codes

different stages along the visual pathway represent increasingly more abstract properties of the stimulus

different visual subsystems encode different properties

(e.g. obects vs. motion)

1. Limited channel capacity

2. Creating more efficient representations by removing redundant signals

neurons have a small range of response levels

redundancy

• nearby locations typically have the same light level

representing how stimuli deviate from expectations

coding a color by how it differs from gray or a face by how it differs from an average face

developmental changes and perceptual learning

decode the the sensory signals to estimate the stimulus by combining evidence from the senses with prior expectations

example of a prior: assume lighting is from above to interpret shading cues

for focusing light to form image on the retina

thin layer of neurons along back of eye that absorb (receptors), process and transmit information about the light image

small region of retina specialized for finest vision, corresponding to the center of gaze

receptor-free “hole” in retina where ganglion cell axons leave the eye

conversion of physical energy to neural signal

2 branches of dark adaptation curve reveal two mechanisms

Rods : High

Cones : low

Cones : moderate- bright

Rods : dim light

Rods: low

Cones: high

Rods : peak around 20 deg

Cones: peak in fovea

Rods : 1

Cones : 3

Rods : no

Cones : yes

area on the retina to which a cell responds

Center - surrounded

1. Direct receptor input vs. indirect input through horizontal cells

2. on-center vs. off-center

3. Lateral inhibition: inhibition between spatially adjacent cells

1. forward – Receptors to bipolar cells to ganglion cells

2. Lateral – Horizontal cells and amacrine cells

on-center : responds to a bright spot of light that just covers the center

off-center : responds to a dark spot of light that just covers the center

inhibition between spatially adjacent cells

Bipolar cells

1. Straight through: receptor to bipolar to ganglion cell

2. Sideways: via horizontal cells and amacrine cells

on-center cell responds best to a bright spot of light that just covers the center

off-center cell responds best to a dark spot that just covers the center

Simultaneous Contrast

Hermann Grid

Mach bands

Lateral geniculate nucleus (LGN)

so each LGN represents contralateral visual field

(ie opposite side of the world)

magnocellular and parvocellular

These come from both eyes to line up signals

Ganglion cells

Examples : center-surround and monocular

Responds to signals only from one eye

striate cortex or V1

where most LGN cells send their axons

1. Orientation selectivity

2. Binocular

3. Increased selectivity for size

4. Sensitivity to direction of movement

5. simple cells v complex cells

cells receive inputs from both eyes

spatially separated on and off subregions in receptive field, like center-surround cells

same stimulus selectivity as simple cells but no clear subregions within receptive field

1. Retinotopic organization

2. Columnar organization

3. The hypercolumn

spatial layout of retina is preserved by layout of cortex, but

Cortical magnification factor: much more of cortex is devoted to the fovea, much less to peripheral retina

much more of cortex is devoted to the fovea, much less to peripheral retina

at each location cells in different layers have similar properties (e.g. preferred stimulus orientation), while RF’s change systematically as move from one point on the cortex to the next (e.g. preferred orientation changes smoothly from one column of cells to the next)

a functional module (~1mm square of cortex) that processes all orientations, eye combinations, colors, sizes, motion direction for a given location in space

Different areas are specialized for processing different visual tasks at the same time

about 30 distinct visual areas have been identified in primate cortex, suggesting a great deal of visual perception happens after the striate

where pathway (parietal cortex) vs. what pathway (temporal)

In the retina

magnocellular

parvocellular