PSYC 102 - Sensory Neuroscience

perception is

a two-way inferential process

ganglion cells

signal differences in light levels across a small area of the image (contrast)

visual pathway (1)

retina

visual pathway (2)

LGN

visual pathway (3)

primary visual cortex (V1)

rods -> m-ganglion cells ->

MAGNO layers of LGN

cones -> p-ganglion cells ->

PARVO layers of LGN

retinotopy

projections from retina to LGN preserve spatial relationships

anatomical positions of neurons...

reflect spatial relationships in input images

retinal ganglion cells (via the LGN)...

feed in the simple cells of the primary visual cortex

orientation tuning curve

a curve on a graph that shows the average response of an orientation-tuned neuron such as a simple cell to stimuli with different orientations

simple cortical cells

excitatory and inhibitory areas arranged side by side. responds best to bars of a particular orientation

complex cortical cells

responds best to movement of a correctly oriented bar across the receptive field. many cells respond best to a particular direction of movement

end-stopped cortical cells

responds to corners, angles, or bars of a particular length moving in a particular direction

selective adaptation

neurons tuned to specific stimuli change their firing rate (adapt) when exposed to the same stimulus for long periods of time

specificity coding

specific neurons respond to specific stimuli

distributed coding

pattern of firing across many neurons codes specific objects

sparse coding

only a relatively small number of neurons are necessary

easy problem of consciousness

how does neural activity CORRELATE with experience?

hard problem of consciousness

how does neural activity CAUSE experience?

structuralist approach

perceptions are created by combining elements called sensations

gestalt approach

the whole is different than the sum of its parts. perception is NOT built from sensations but are a result of perceptual organization

spatial attention

attention directed to specific locations in space

feature-based attention

direction of attention to a particular feature, anywhere in space

attention...

improves performance

inference

your brain is doing its best to figure out what's out there, given the limited input

convergence ratio of rods and codes to ganglion cells

120 million rods and cones converge to 1 million ganglion cell

there is a ______________ convergence of rods than cones

higher

average of ______ rods to 1 ganglion cell

120 rods

average of ______ cones to 1 ganglion cell

6 cones

cones in fovea...

have roughly 1 to 1 relation to ganglion cells

rod system in periphery is...

more sensitive to low light (higher convergence, lower acuity)

cone system in fovea is...

less sensitive (lower convergence, higher acuity)

all-cone foveal vision

results in high visual acuity

one-to-one wiring...

leads to ability do discriminate details

what is the trade-off for convergence and detail?

cones need more light to drive a ganglion cell than rods

if two spots of light stimulate the same ganglion cell...

the brain CANNOT tell them apart

if two spots stimulate different ganglion cells...

it is at least POSSIBLE for you to tell them apart

experiments with eye of limulus

ommatidia (light sensitive cells) allow recordings from a single receptor

light shown into a single receptor led to rapid firing rate of nerve fiber

adding light into neighboring receptors led to reduced firing rate of initial nerve fiber

lateral plexus

the structure through which lateral inhibition is occurring

hermann grid

illusory spots of light. a display that results in the illusion of dark areas at the intersection of two white "corridors." this perception can be explained by lateral inhibition

simultaneous contrast

illusion of changed brightness or color due to the effect of the adjacent area. an area that is of the same physical intensity appears lighter when surrounded by darker area, darker when surrounded by lighter area

what is the purpose of on/off receptive fields?

for ganglion cells to signal differences in light across a small image. they don't just signal the presence or absent of light in a region of space or the absolute magnitude of light. the firing of a given cell carries information about the local features (i.e. changes) in a region of space

visual pathway of rods

rods -> M-ganglion cells -> MAGNO layers of LGN -> V1

visual pathway of cones

cones -> P-ganglion cells -> PARVO layers of LGN -> V1

cortical magnification

the amount of cortical area devoted to a specific region in the visual field. the fovea has more "cortical space" than expected and provides extra processing for high-acuity tasks

center-surround antagonism

output of center-surround receptive fields changes depending on area stimulated. highest response when only excitatory area is stimulated, lowest response when only the inhibitory area is stimulated, intermediate responses when both areas are stimulated

hubel and wiesel's classic study of receptive fields

signals from the retina travel through the optic nerve to the LGN. primary visual receiving area in the occipital lobe then through 2 pathways to the temporal lobe and parietal lobe

major function of LGN...

regulate neural information from the retina to the visual cortex. signals are received from the retina, cortex, brain stem, and thalamus. signals are organized by the eye, receptor type, and type of environmental information

spatial arrangement of ganglion cells...

feed into a simple cell that creates the receptive field of the simple cell

ocular dominance columns

neurons in primary visual vortex initially respond best to one eye. neurons with the same preference are organized into columns. the columns alternate in a left-right pattern every .25 to .50 mm across the cortex

optic nerve fiber (ganglion cell)

center-surround receptive field. responds best to small spots, but will also respond to other stimuli

lateral geniculate nucleus (LGN)

the part of the thalamus that receives information from the optic tract and sends it to visual areas in the occipital cortex. center-surround receptive fields very similar to the receptive field of a ganglion cell

dorsal pathway (where)

rods -> M-ganglion cells -> MAGNO layer of LGN -> V1 -> dorsal pathway -> parietal lobe

ventral pathway (what)

cones -> P-ganglion cells -> PARVO layer of LGN -> Bv1 -> ventral pathway -> temporal lobe

dorsal pathway

where/how pathway - parietal lobe

ventral pathway

what pathway - temporal lobe

object discrimination problem

a problem in which the task is to remember an object based on its shape and choose it when presented with another object after a delay. associated with research on the what processing stream

landmark discrimination problem

a problem in which the task is to remember an object's location and to choose that location after a delay. associated with research on the where processing stream

removing temporal lobe tissue...

resulted in problems with the object discrimination task (what pathway - ventral)

removing the parietal lobe tissue...

resulted in problems with the landmark discrimination task (where pathway - dorsal)

feature detectors

neuron that responds selectively to a specific feature of the stimulus such as orientation or direction of motion

contrast threshold

the smallest amount of contrast required to detect a pattern

fatigue or adaptation to stimulus causes...

- neural firing rate to decrease

- neuron to fire less when stimulus immediately presented again

selective adaptation means...

only those neurons that respond to the specific stimulus adapt

using selectie adaptation

1. measure sensitivity to range of one stimulus characteristic

2. adapt to that characteristic by extended exposure

3. re-measure the sensitivity to range of the stimulus characteristic

neural plasticity

the ability of the brain to change in response to experience. when animals are reared in environments that contain only certain types of stimuli, neurons responsive to rearing stimuli become predominant

inferotemporal (IT) cortex

an area of the brain outside V1 (the striate cortex), involved in object perception and facial recognition. part of the cerebral cortex in the lower portion of the temporal lobe, important in object recognition

prosopagnosia

inability to recognize faces

fusiform face area (FFA)

an area in the temporal lobe that contains many neurons that respond selectively to faces

grandmother cell

one cell that encodes each "highest level" object. a neuron that responds only to a specific stimulus

sensory code

representation of perceived objects through neural firing. how neurons represent various characteristics of the environment

problems with specificity coding

1. too many different stimuli to assign specific neurons

2. most neurons respond to a number of different stimuli

inverse projection problem

an image on the retina can be caused by an infinite number of objects

viewpoint invariance

the ability to recognize an object seen from different viewpoints

continuation (gestalt principle)

images are seen in ways that produce smooth continuation. continuous shapes viewed as single segmented objects. (ex. bounded rope)

proximity/similarity (gestalt principle)

things that are near each other are grouped together. things that are similar are grouped together

common fate (gestalt principle)

humans perceive visual elements that move in the same speed and/or direction as parts of a single stimulus. things moving in the same direction are grouped together

common region (gestalt principle)

elements in the same region tend to be grouped together

uniform connectedness (gestalt principle)

connected region of visual properties are perceived as single unit

good continuation

a gestalt grouping rule stating that two elements will tend to group together if they seem to lie on the same contour. helps us perceive things as a continuous object versus a bunch of disconnected or overlapping objects

synchrony

elements occurring at the same time are seen as belonging together

meaningfulness or familiarity

things are more likely to form groups if the groups appear meaningful or similar

covert attention

attention without looking

saccades

quick eye movements from one fixation to another

bottom-up processing

allowing stimulus itself to shape perception. analysis that begins with the sensory receptors and works up to the brain's integration of sensory information

top-down processing

using our background knowledge and expectations to interpret what we see. we form information processing guided by higher-level mental processes, as when we construct perceptions drawing on our experience and expectations

bottom-up factors

characteristics of a stimulus scene such as color, contrast, orientation

top-down factors

task or goals, experience, attention (where to attend), what features to attend to

stimulus salience

bottom-up factors that determine attention to elements of a scene. examples are color, contrast, and orientation

saliency map

a "map" of a visual display that takes into account characteristics of the display such as color, contrast, and orientation that are associated with capturing attention

endogenous spatial attention

a form of top-down control of spatial attention in which attention is VOLUNTARILY directed toward the site where the observer anticipates a stimulus will occur

exogenous spatial attention

a form of bottom-up spatial attention in which attention is INVOLUNTARILY and reflexively directed toward the site at which a stimulus has abruptly appeared