Chapter 4 Flashcards
Optic chiasm
An x-shaped bundle of fibers on the underside of the brain, where nerve fibers activated by stimulation of one side of the visual field cross over to the opposite side of the brain.
Visual field
The visual field is determined based on where the person is fixating; anything to right of the point of central focus is the right visual field (processed by the left hemisphere), and anything to the left is the left visual field (processed by the right hemisphere). Importantly, both eyes can see both visual fields.
Lateral geniculate nucleus (LGN)
The nucleus in the thalamus (of each hemisphere) that receives inputs from the optic nerve and, in turn, communicates with the cortical receiving area for vision. Around 90% of the signals from the retina proceed to the LGN. Neurons in the LGN have center-surround receptive fields.
Superior colliculus
An area in the brain that is involved in controlling eye movements and other visual behaviors. This area receives about 10% of the ganglion cell fibers that leave the eye in the optic nerve.
Function of the LGN (theories)
- Regulate neural information as it flows from the retina to the cortex (comes from the observation that the signal sent from the LGN to the cortex is smaller than the input the LGN receives from the retina)
- The feedback information the LGN receives back from the brain may play a role in determining which information is sent up to the brain (the LGN is that it receives more signals from the cortex than from the retina)
Visual receiving area
The area of the occipital lobe where signals from the retina and LGN first reach the cortex. The visual receiving area is also called the striate cortex, because it has a striped appearance when viewed in cross section, or area V1 to indicate that it is the first visual area in the cortex.
Simple cortical cells
A neuron in the visual cortex that responds best to bars of a particular orientation. By flashing moving lines of light on different places in the retina, Hubel and Wiesel (1965) found cells in the striate cortex with receptive fields that, like center-surround receptive fields of neurons in the retina and LGN, have excitatory and inhibitory areas. However, these areas are arranged side by side rather than in the center-surround configuration. There are neurons that respond to all of the orientations that exist in the environment.
Orientation tuning curve
A function relating the firing rate of a neuron to the orientation of the stimulus. It is determined by measuring the responses of a simple cortical cell to bars with different orientations.
Complex cells
A neuron in the visual cortex that responds best to moving bars with a particular orientation. Further, many complex cells respond best to a particular direction of movement. Because these neurons don’t respond to stationary flashes of light, their receptive fields are indicated not by pluses and minuses but by outlining the area that, when stimulated, elicits a response in the neuron.
End-stopped cells
A cortical neuron that responds best to lines of a specific length or to moving corners or angles (moving in a particular direction).
Feature detectors
A neuron that responds selectively to a specific feature of the stimulus such as orientation or direction of motion.
Selective adaptation
When we view a stimulus with a specific property, neurons tuned to that property fire. The idea behind selective adaptation is that this firing causes neurons to eventually become fatigued, or adapt. Typically, sensitivity to the exposed stimulus is decreased.
Physiology of selective adaptation
The adaptation causes two physiological effects:
1. the neuron’s firing rate decreases,
2. the neuron fires less when that stimulus is immediately presented again.
Selective adaptation explanation
According to this idea, presenting a vertical line causes neurons that respond to vertical lines to respond, but as these presentations continue, these neurons eventually begin to fire less to vertical lines. Adaptation is selective because only the neurons that were responding to verticals or near-verticals adapt, and neurons that were not firing do not adapt.
Contrast threshold
The intensity difference between two areas that can just barely be seen. This is often measured using gratings with alternating light and dark bars.
How to measure selective adaptation
- Measure a person’s contrast threshold
- Adapt the person to one orientation by having the person view a high-contrast adapting stimulus for a minute or two. In this example, the adapting stimulus is a vertical grating
- Remeasure the contrast threshold of all the test stimuli presented in step 1
Selective adaptation vs. orientation selectivity curve
Adaptation selectively affects only some orientations, just as neurons selectively respond to only some orientations. The selective adaptation curve and the orientation tuning curve for a simple cortical neuron are very similar. This similarity support the idea that feature detectors play a role in perception. The selective adaptation experiment is measuring how a physiological effect (adapting the feature detectors that respond to a specific orientation) causes a perceptual result (decrease in sensitivity to that orientation).
Selective rearing
The idea behind selective rearing is that if an animal is reared in an environment that contains only certain types of stimuli, then neurons that respond to these stimuli will become more prevalent. According to this idea, rearing an animal in an environment that contains only vertical lines should result in the animal’s visual cortex having simple cells that respond predominantly to verticals.
Neural plasticity or Experience-dependent plasticity
A process by which neurons adapt to the specific environment within which a person or animal lives. This is achieved when neurons change their response properties so they become tuned to respond best to stimuli that have been repeatedly experienced in the environment.
Selective adaptation vs. selective rearing
- Adaptation is a short-term effect. Presenting the adapting orientation for a few minutes decreases responding to that orientation.
- Selective rearing is a longer-term effect. Presenting the rearing orientation over a period of days or even weeks keeps the neurons that respond to that orientation active. Meanwhile, neurons that respond to orientations that aren’t present are not active, so they lose their ability to respond to those orientations.