Vision Flashcards
Describe why and how your eyes accommodate for close vision
Light rays from close objects diverge and require more refraction to focus them at the back of the eye. The eye accommodates for close vision by tightening the ciliary muscles and allowing the pliable crystal lens to become more rounded which gives us an extra umph with focusing
what is subfield antagonism?
Subfield antagonism is a term used to explain the behavior of a bipolar cell or ganglion cell when presented with a uniform stimulus across its entire receptive field (i.e. bright/dark stimulus on both center and surround). Can elaborate more..
What are two advantages of having a center-surround receptive field organization in the retina?
1) optimizes our capacity for edge-detection/feature-detection which is essential for identifying visual objects (arranged to detect contrast boarders at discrete spatial positions)
2) mutual antagonism between subregions reduces redundancy in signals sent downstream
What is spike-triggered averaging and what is it used for?
Spike-triggered averaging is a technique used to decipher the nature of stimuli that a cell preferentially responds to. In studies of the visual system, it is used to determine the receptive field and sign of stimulus of a particular neuron. This technique involves presenting a series of frames depicting different stimuli (the visual stimulus presented is sometimes sampled from a gaussian distribution e.g. Gaussian noise) while the activity of the neuron of interest is being recorded. The experimenter then averages the frames the preceded a spike in order to reconstruct the stimulus that evoked the recorded response over several trials.
What is cross-correlation analysis and what is it used for?
Cross-correlation analysis is used to determine two cells are functionally correlated/synaptically connected. It involves measuring spike timing of reference neuron A and then spike timing of reference neuron B and see if their spike timing patterns are correlated (generate a cross-correlogram to visualize spike timing patterns)
What is push and pull?
Terms used to describe the behavior of thalamic neurons in response to different type of stimuli.
Push - excitation in response to a stimulus of the appropriate sign (e.g. dark center in an OFF-center relay cell)
Pull - inhibition in response to a stimulus of the inappropriate sign (e.g. bright center on OFF-center relay cell)
Explain the neural mechanism responsible for the push-pull behavior of LGN cells?
LGN relay cells have ON/OFF center-surround receptive fields just like retinal ganglion cells do. For example, an OFF-center LGN relay cell is excited by a dark stimulus in the center and a bright stimulus int he surround, this behavior is described as the “push”. However, when the OFF-center LGN relay cell is presented with a bright stimulus in the center, we observe an inhibitory current which is stemming from local feedforward inhibition supplied by a nearby inhibitory interneuron of the opposite sign preference.
(a) Explain the concept of divergence/convergence in relation to LGN visual processing and (b) explain how the benefit and cost of this process in terms of spatial resolution and signal to noise ratio. (c) Describe how the push-pull behavior of LGN neurons can help overcome some of the issues arising from divergence/convergence.
(a) Convergence refers to the fact that multiple retinal ganglion cells project their input onto a single LGN relay cell - i.e. information about the various visual features conveyed by each retinal ganglion cell converges on the same LGN relay cell. Conversely, divergence refers to the fact that a single retinal ganglion cell can project to multiple different LGN relay cells.
(b) The main disadvantage of this divergence/convergence of retinal input in the LGN results in the visual image being upsampled and interpolated simply due to the fact that there are twice as many relay cells as there are ganglion cells - this upsampling and interpolation inevitably produces image blur. However, the main advantage of divergence/convergence is that a given LGN relay cell is able to take an average of all the retinal ganglion cell inputs which effectively reduces the amount of noise contaminating the visual signal.
(c) While the push, i.e excitatory response of LGN relay cells in response to the appropriate stimulus luminance contrast already aids our ability to detect edges, the pull, i.e. the inhibitory response of a neighboring LGN inhibitory interneuron with the opposite stimulus contrast preference is gives us a boost in contrast-boarder detection that effectively sharpens the otherwise blurred image that results from upsampling and interpolating the retinal input.
Explain how the receptive fields of LGN neurons contribute to the receptive field of a simple V1 cell.
Simple visual cortical cells have receptive fields that can be described by simple geometric shapes. Hubel and Wiesel determined that these cortical cells mainly reside in layer 4 of the visual cortex which is the main thalamic input layer. Using cross-correlation analysis, they demonstrated that simple cortical cells receive input LGN cells that are aligned in visual space and share the same preference for luminance contrast. In other words, an LGN cell is connected to a simple cortex cell when it’s center was in alignment with the subregion of the downstream simple cell.
What are orientation and hypercolumns? Explain their function.
Orientation columns are vertically aligned cells across the cortical depth (cortical layers) that share the same preference for stimulus orientation and have overlapping visual fields.
A hypercolumn is a region of cortex that represents one degree of visual space in which a given stimulus attribute, for example, orientation, is analyzed for all possible permutations
What are two main visual processing pathways in the cortex and what sort of information do they encode?
There is a dorsal “where” and a ventral “what” pathway in vision. In the dorsal or “where” pathway, visual information from the primary and secondary visual cortices is projected to the middle temporal area and from there into various parietal and motor areas. This pathway processes directional and orientational cues, and is concerned with localizing visual objects in space. The ventral or “what” pathway projects information from the primary and association visual cortices to various temporal areas, and is involved in identifying visual objects and therefore processes features such as form and color.
Describe the neural circuit that gives rise to directional motion selectivity to a visual cortical neuron.
A neuron’s selectivity for direction of movement depends on the response latencies of presynaptic neurons relative to the onset of a stimulus. For example, a cortical neuron with selectivity for movement from left to right receives serial inputs from presynaptic neurons a, b, c, d, and e. If neurons a and b have longer response latencies than neurons d and e, when the stimulus moves from left to right, neurons a and b are activated first but because their response latencies are longer, their inputs arrive at the target cell superimposed with the inputs from neurons d and e and the summated inputs cause the target cell to fire. If a stimulus were to move in the opposite direction, the presynaptic signals would arrive at the target cell at different times and consequently the target cell’s threshold for firing would not be reached and there would be no response.
Describe the visual circuit that allows us to perceive depth.
Depth is computed from the relative positions of objects placed at different distances from the observer. An object that lies in the plane of fixation produces images at corresponding positions on the two retinas. Objects that lie in front of or behind the plane of fixation fall on slightly different locations in the two eyes, which is a phenomenon called binocular disparity. Individual neurons can be selective for a narrow range of disparities and therefore depth.
