chapter 3 (continued) Flashcards
neural convergence/convergence for short
when a number of neurons synapse onto a single neuron - a lot of convergence happens in the retina because the eye has 126 million photoreceptors but only 1 million ganglion cells
rod convergence
many many rods converge onto a single ganglion - about 120 rods to 1 ganglion - this makes rods very sensitive because it takes less incoming light to stimulate the ganglion cell since that light is received by many rods all converging onto the ganglion
cone convergence
very few cones converge onto a single ganglion - about 6-1 outside the fovea and 1-1 inside the fovea - less convergence means that cones have very high acuity since as opposed to rods, each cone stimulates its own ganglion cell, meaning when that cell fires, we know exactly where the signal is coming from (whereas in rods, any number of rods could fire the same ganglion)
receptive field
each ganglion cell responds to stimuli in specific parts of the retina - found by hartline studying single ganglion cells in the opened eye cup - retina of a frog - found that cells only responded when a specific area of the retina was stimulated
centre surround receptive fields
receptive fields that respond differently to light in their centre than light in the surrounding area - can be excitatory centre, inhibitory surround or inhibitory centre, excitatory surround
excitatory area
area of the receptive that increases firing - can be either centre or surround
inhibitory area
area of the receptive field that inhibits firing - can be either centre or surround
centre surround antagonism
stimulating centre and surround areas simulateously decreases responding of the neuron, compared to stimulating the excitatory area alone - this is because one area is inhbitory and one is excitatory so they cancel each other out
lateral inhibition
inhibition that is transmitted across the retina (by horizontal and amacrine cells) happens when excitatory synapses in horizontal and amacrine cells have inhibitory synapses on downstream cells
cell example of lateral inhibition
neural circuit in which all receptors are excitatory and the receptors on the surround converge on neurons that have inhibitory lateral connections to one central neuron that sums all the responses - when light hits the centre, the receptors activate the centre neuron (excitatory) but when light hits the surround the surround neurons activate - when these neurons are excited they inhibit the centre neuron that they are connected to, and decrease the overall response -
edge enhancement
an increase in perceived contrast at border between regions of the visual field - edges look more distinct so we can see them more easily
chevreul illusion
perceived light and dark bands at the border between two colours of contrasting lightness that makes the edge between the colours look sharper - this is not present in the physical stimuli
Mach bands
light and dark bands created at fuzzy borders - gradual transition from light to dark
how do centre surround fields in ganglion cells explain the edge enhancement in the chevreul and mach illusions?
when we look at two areas of light vs dark next to eachother, different ganglion cells receive different input - some only receive input from the light area, and some from the dark. In centre surround ganglion cells, the cells that receive input from the light side are more inhbiited because their entire field is illuminated, including the inhibitory surround field. cells on the border are slightly less inhibited, because part of their inhibitory field is in the dark side, not illuminated, resulting in a light band on the border. cells that receive input from the dark side are less inhibited because none of their receptive area is illuminated. cells on the border of the dark side are slightly more inhbited, because part of their inhibitory surround is in the lighter region, resulting in a dark band on the border
**all cells have baseline firing*
preferential looking (PL) technique
test of infant visual acuity measured by how long infants look at stimuli - we assume that if an infant looks preferentially at one of two stimuli, they can tell the difference between them, whereas, if they look at both stimuli equally we assume they cannot tell the difference
visual evoked potential
an electrical response generated by the visual system when detail is detected - can be used to measure whether or not an individual (infants) can detect detail
infant visual acuity
visual acuity is poorly developed at birth and increases rapidly over the first 6 to 9 months - full visual acuity is achieved sometime after 1 year of age - this is because their cones are not fully developed ( too fat (so not tightly packed, gaps) with small outer segments resulting in less visual pigment)
limulus
type of horseshoe crab that is used for research on lateral inhibition because its receptors are large enough so that stimulation can be applied to individual receptors
ommatidia
a structure in the eye of the limulus that contains a small lens located directly over a visual receptor - they limulus eye is made up of hundreds of these ommatidia
specificity coding
hypothesis that we have specialized neurons that respond to only one concept or stimulus - ex grandmother neuron
sparse coding
theory that particular stimuli are represented by a pattern of firing of only a small group of neurons (most remain silent)
population coding
theory that our experiences are represented by the pattern of firing across a large number of neurons - this allows are large number of stimuli to be represented because large groups of neurons can create a huge number of different patterns
modularity
the idea that specific brain areas are specialized to respond to specific types of stimuli or functions - each area is called a module - studied by recording brain responses in neurologically normal humans
distributed representation
idea that the brain represents information in patterns distributed across the cortex, not just one brain area - this approach focuses on the activity in multiple brain areas and connections between those areas
structural connectivity vs functional connectivity
structural connectivity: “road map” of fibres connecting different areas of the brain
functional connectivity: neural activity associated with a particular function that is flowing through this structural network
the hermann grid illusion
when we look at a grid of black squares, we see dark dots at the intersections between the squares - this effect goes away when we look directly at them
hermann grid illusion explanation
we see dark spots in our periphery because at the intersections, more light falls on the inhibitory surround area of our receptive fields than it does for any other area - compared to the receptive fields between squares, there is less excitatory stimulation, resulting in darker areas in the intersections
the dark spots disappear as soon as we look directly at them because when we look directly, we use our central vision, which has the greatest acuity - ganglion cells in this area (fovea) have smaller receptive fields with less inhibitory inputs, resulting in less inhibition of the centre by the surround, and the dark spots disappear
