Pathway from Retina to Cortex

Retina → Lateral geniculate nucleus (LGN) → V1 V1: primary visual cortex (occipital lobe), also called striate cortex From V1: pathways to temporal and parietal lobe → frontal lobe Here, we focus on processing in (pathway from retina to) V1. not directly from eye to cortex early on process is based on visual field

Lateral geniculate nucleus processing

LGN cells have center-surround receptive fields. stimulating at sensitive area of the cells, decreases further away Major function of LGN is to regulate neural signals from retina to visual cortex. Signals are received from retina, cortex, brain stem, and thalamus. Signals are organized by eye, receptor type, and type of environmental information

Maps: Representing Spatial Layout

Retinotopic map - each place on retina corresponds to a place on LGN Determining retinotopic maps - record from neurons with an electrode that penetrates the LGN obliquely Stimulating receptive fields on retina shows location of corresponding neuron in LGN gradual shift in position in LGN

The Neural Map on the Striate Cortex (Area V1)

gradually changes in sensitivity to orientation

Receptive fields

which neurons are active in specific areas of light on retina

fMRI - The Neural Map on the Striate Cortex (Area V1)

Functional magnetic resonance imaging (fMRI) Hemoglobin carries oxygen and contains a magnetic ferrous molecule Brain activity takes up oxygen: hemoglobin becomes more magnetic. need blood flow in more active areas, these areas become more magnetic fMRI: inferring localized brain activity by measuring changes in magnetic response of hemoglobin. Subtraction technique is used.

Cortical magnification from MRI

Signals from fovea reach larger cortical space (more cells) than more peripheral parts of retina more cells processing in fovea, most detailed processing here where you look you have more acuity

Visual Cortex - Location columns

Receptive fields at same location on retina are within a column. orientation columns are within location columns

Visual Cortex - Orientation columns

Neurons within columns fire maximally to the same orientation of stimuli. Adjacent columns change preference in an orderly fashion. One millimeter across cortex represents entire range of orientations. Spike is an action potential, how active a neuron is vertical line most active some cells are tilted, they prefer tilted lines, looks straight to them

Ganglion cell - Receptive Fields of Neurons in the Visual Cortex

Center-surround receptive field. Responds best to small spots but will also respond to other stimuli.

Lateral Geniculate

Center-surround receptive fields very similar to the receptive field of a ganglion cell.

Simple cortical

Excitatory and inhibitory areas arranged side by side. Responds best to bars of a particular orientation

Complex cortical

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

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

Contextual modulation - “Flexible” Receptive Fields

Perceptual system needs to be flexible. Kapadia’s (2000) research Response to stimulation within receptive field affected by ‘what’s happening’ outside receptive field.

Higher-Level Neurons

Inferotemporal (IT) cortex Prosopagnosia Fusiform face area more active when specific complex shapes are present

Specificity coding

"Grandmother cell” hypothesis Cells in hippocampus respond to concepts such as Halle Berry. Problems Too many different stimuli (people we know, animal names etc) to assign specific neurons Most neurons respond to several different stimuli Cell deaths, common theoretically if cell died would lose memory

Population coding

Pattern of firing across many neurons codes specific objects Large number of stimuli can be coded by a few neurons.

Sparse Coding

How many neurons are needed for an object in distributed coding? Sparse coding: only a relatively small number of neurons are necessary This theory can be viewed as a midpoint between specificity and distributed coding.

Modularity

Module: brain structure that processes information about specific stimuli Rolls measured response neurons in inferotemporal (IT) cortex in monkeys Responds best to faces with little response to non- face stimuli Temporal lobe damage in humans results in prosopagnosia. Rolls & Tovee (1995) if this area of the brain becomes damaged won't recognise faces

Areas for Places, and Bodies in Human Brain

Evidence from humans using fMRI and the subtraction technique show that: Parahippocampal place area (PPA) responds most to spatial layout Extrastriate body area (EBA) responds most to pictures of bodies and body parts

selective Adaptation

Neurons tuned to specific stimuli fatigue when exposure is long. • Fatigue or adaptation to stimulus causes • Neural firing rate to decrease • Neuron to fire less when stimulus immediately presented again • ‘Selective’ means that only those neurons that respond to specific stimulus adapt. Measure sensitivity to range of stimulus with one same characteristic Adapt to that characteristic by extended exposure Re-measure the sensitivity to range of stimulus characteristic

Gratings selective adaptation

Alternating light and dark bars Angle relative to vertical can be changed to test for sensitivity to orientation. Difference in intensity can be changed to test for sensitivity to contrast Measure contrast threshold by decreasing intensity of grating until person can just see it. Calculate the contrast sensitivity by taking 1/threshold. If threshold is low, person has high contrast sensitivity.

Selective Rearing

Animals are reared in environments that contain only certain types of stimuli. Neurons that respond to these stimuli will become more predominate due to neural plasticity. Blakemore and Cooper (1970) showed this by rearing kittens in tubes with either horizontal or vertical lines. Both behavioral and neural responses showed development of predominant sensitivity to these environmental stimuli.