Chapter 3 Study Guide Spatial Vision 1 Key terms Aliasing misperception of a grating due to undersampling o Aliasing is the misperception of a grating due to undersampling When looking at gratings the visual system samples the grating discretely via the array of receptors at the back of the retina If the receptors are spaced such that the lightest and darkest parts of the grating fall on separate cones the observer can detect the grating However if the lightest and darkest parts of the grating both fall on the same cones then the grating will be aliased and appear gray Amplitude how since waves are defined Also defined by frequency Sine waves are the building blocks for understanding visual and auditory perception Acuity the smallest spatial detail that can be resolved Contrast the difference in luminance between an object and the background or between lighter and darker parts of the same object spatial frequency the number of cycles of a grating per unit of visual angle usually specified in degrees visual angle the angle subtended by an object at the retina topographical mapping The orderly mapping of the world in the lateral geniculate nucleus and the visual cortex orientation tuning The tendency of neurons in striate cortex to respond optimally to certain orientations and less to others Phase the relative position of the grating its position within the receptive field cortical magnification The amount of cortical area usually specified in millimeters devoted to a specific region e g 1 degree in the visual field simple cell a cortical neuron with clearly defined excitatory and inhibitory regions complex cell a neuron whose receptive field characteristics cannot be easily predicted by mapping with spots of light pattern analyzers The human visual cortex contains pattern analyzers that are specific to spatial frequency and orientation Adaptation a reduction in response caused by prior or continuing stimulation 2 What are receptive fields and size to perceive texture receptive field is the region on the retina in which visual stimuli influence a neuron s firing rate The receptive field has to be smaller than 1 2 the period of the sine wave to perceive texture 3 Snellen s 1862 method for designating Herman Snellen constructed a set of block letters for which the letter as a whole was five times as large as the strokes that formed the letter Defined visual acuity as a the distance at which a person can just identify the letters the distance at which a person with normal vision can just identify the letters 2 Hubel and Wiesel s experiment as seen in the video on the slides a Most fundamental discovery was that neurons in striate cortex respond to stripes not stars The receptive fields of striate cortex neurons are not circular as they are in the retina and LGN Rather they are elongated responding much more vigorously to bars lines edges and gratings collection of lines than to round circles of light b An individual neuron will not respond equivalently to just an old stripe in its receptive field It responds best when the line or edge is at its rights orientation and hardly at all when the line is tilted more than 30 degrees away from the optimal orientation c Important discovery made Hubel and Wiesel was that many cortical cells respond especially well to moving lines bars edges and gratings 2 What do retinal ganglion cells respond to Retinal ganglion cells respond vigorously to spots of light Each ganglion cell also responds well to certain types of stripes or grating a Retinal ganglion cells are tuned to spatial frequency each cell responds best to a specific spatial frequency that matches its receptive field size and it responds less to both higher and lower spatial frequencies medium frequency yields strong response i When the spatial frequency of the grating is too low the ganglion cell responds weakly because part of the fat bright bar of the grating lands in the inhibitory surround damping the cell s response low frequency yields weak response ii When the spatial frequency is too high the ganglion cell responds weakly because both dark and bright stripes fall within the receptive field center washing out the response high frequency yields weak response 2 Know the entire pathway of a visual signal starting at the retina and where the signal converges a Retina optic nerve optic chiasm optic tract LGN optic radiation visual cortex Notes The lateral geniculate nucleus is a nucleus in the midbrain that shares connections with both the retina and visual cortex Neurons in the bottom two layers are physically larger than those in the top four layers o Bottom two layers are called Magnocellular o Top four layers are called parvocellular layers Sine waves are the building blocks for understanding visual and auditory perceptions o Defined by amplitude and frequency o Shape is sinusoidal with rounded beaks Visual Processing o Motion LGN Magnocellular layers receive input from M ganglion cells in the retina o Color Form LGN Parvocellular layers receive input from P ganglion cells Striate cortex primary visual cortex V1 the area of the cerebral cortex of the brain that receives direct input from the lateral geniculate nucleus LGN as well as feedback from other brain areas and is responsible for processing visual information