Visual perception II: Higher-level vision Flashcards
(35 cards)
What is the (main) visual pathway?
The visual pathway refers to the anatomical structures responsible for the conversion of light energy into electrical action potentials that can be interpreted by the brain. It begins at the retina and terminates at the primary visual cortex.
What does the main visual pathway consist of?
The visual pathway consists of the retina, optic nerves, optic chiasm, optic tracts, lateral geniculate bodies, optic radiations, and visual cortex.
What are two visual pathways?
The visual pathway is segregated into two distinct streams—ventral and dorsal.
The ventral stream (what) processes information about object identity, sampling high-resolution/focal spaces. The dorsal stream (where) either processes information about object location or executes movements under visual control, sampling nearly all of space with reduced foveal bias.
How can retina be divided?
The retina can be divided into four quadrants: the upper and lower nasal retina and the upper and lower temporal retina. The nasal retina of the left/right eye and the temporal retina of the right/left eye receives visual input from the left/right visual field. The upper parts of the retina receive visual stimuli from the inferior visual field, the lower part of the retina is receive visual stimuli from the upper visual field.
What is lateral geniculate nucleus?
The thalamic nucleus that is the main target of axons of the optic tract.
What does LGN do?
The LGN receives visual information transmitted from the retina through the optic nerve in the retinogeniculate pathway, and then sends information on to the primary visual cortex (V1) via the geniculocortical pathway.
Its functions include attentional modulation, temporal decorrelation, and binocular facilitation or suppression via monocular gain modulation.
What happens in the optic chiasm?
The nasal fibers of each eye cross the midline to join the temporal fibers of the contralateral eye. The visual input from the right half of the visual field will travel to the left hemisphere of the brain via the left optic tract, while all left visual field information to the right hemisphere of the brain via the right optic tract.
How many layers does LGN have? What are they?
LGN has six layers that are distinguished on the response properties (magnocellular vs. parvocellular and contralateral vs. ipsilateral eye)
What are the characteristics of magnocells?
Magnocells code for achromatic vision (bright vs. dark), they respond linearly increasing to the intensity of the light, the code for motion vision.
What are the characteristics of parvocells?
Parvocells code for chromatic vision (colour contrasts), they respond to more stimulus, not so quick, they code for form vision.
What are koniocells (“sand” cells)?
Koniocells are dispersed between the magno- and parvolayers of LGN and code for blue-yellow pathway.
What are the principles of opponent colour system?
Colors are determined by the proportional excitation of three cone types (short - blue, medium - green, long - red).
The levels of excitation of each cone type define color space.
To calculate the opponent process tristimulus values from the LMS color space, the cone excitations are compared:
- the luminous opponent channel (L + M, sometimes + rods)
- the red–green opponent channel (L - M)
- the blue–yellow opponent channel (S - (L + M))
What types of cells are there in V1?
Simple cells, complex cells, and end-stop cells.
What are the characteristics of simple cells?
Simple cells are one-dimensional — they only respond to orientation. They have clearly defined inhibitory and excitatory areas. Their firing rate is directly proportional to the intensity of the stimulus (respond maximally to stimuli that match the preferred frequency).
What is Gabor filter?
Gabor filter is a model of simple cell response in V1, a sinusoid multiplied with a Gaussian distribution curve that can be oriented at different angles.
The Gabor filter model is based on the idea that these cells respond selectively to specific orientations and spatial frequencies of visual stimuli. A Gabor filter extracts these specific features from an image and gives a set of complex values (the amplitude and phase of the filtered image) as an output. These squared complex values represent the energy or magnitude of the filtered image — the response of the simple cell to a visual stimulus. This process can be applied to an image multiple times, each with a different orientation and spatial frequency, to simulate the selectivity of simple cells to different visual features.
What are the characteristics of complex cells?
Complex cells have no definied inhibitory and excitatory areas and respond to a feature (orientation, movement in particular direction) no matter where in the receptive field it is.
What are the characteristics of end-cells?
End-cells (hypercomplex cells) are more of a subclass of simple and complex cells and have properties of both.
They respond to a line of a certain length or to a corner of a larger stimulus, and show reduced or absent response when the line or corner doesn’t reach a certain point or is extended beyond it (=> the outside of the receptive field affects the firing of a cell). They play crucial role in detecting borders of the luminance.
What is classical receptive field?
A classical receptive field (CRF) is a small, localized area within area of the visual field where a change in the stimulus will result in a change in the firing rate of the visual cortex neuron.
What is non-classical receptive field?
A non-classical receptive field (non-CRF) describes the situation when a neuron in the visual cortex responds to a stimulus that is outside its CRF. The reason that we get these effects is
- the long-range horizontal (lateral) connections between neurons in the V1, and
- top-down connections from higher-order visual areas projecting back to V1
What types of non-CRF are there?
The non-CRF can be either suppressive or facilitatory
- A suppressive non-CRF: a stimulus in one location reduces the firing rate of a neuron when a stimulus is presented in another location.
- A facilitatory non-CRF: a stimulus in one location increases the firing rate of a neuron when a stimulus is presented in another location.
cf. lateral inhibition — the activity of a neuron in the visual cortex is influenced by the activity of neurons in its surroundings. It helps to enhance contrast and improve the ability of the visual system to detect edges in the visual scene.
What is contextual modulation?
Contextual modulation is the phenomenon where the perceived properties of a stimulus, such as its color, shape, or size, are influenced by the surrounding context or background. So, the same stimulus can be perceived differently depending on the context in which it is presented. It can be either excitatory or inhibitory and can occur both is early stages (V1) and higher stages (V2, V4).
What topographical models of the visual cortex are there?
Retinotopic mapping: the visual cortex is organized in a way that maps onto the layout of the retina, with adjacent regions of the visual cortex corresponding to adjacent regions of the retina.
Feature maps: the visual cortex is organized into “maps” (coding for color, orientation, spatial frequency, ocular dominance, space)
Hierarchical models: the visual cortex is organized into a hierarchy of processing stages, with each stage being responsible for a different level of visual processing (from early feature detection to complex object recognition)
The Hubel & Wiesel ice-cube model: the V1 is organized into columns that are sensitive to specific features such as orientation, spatial frequency and direction of motion.
What are the higher stages of visual cortex?
The visual cortex is organized into several hierarchical stages, with each stage receiving input from lower stages and processing increasingly complex visual information.
V3: orientation, also motion and depth
MT (Middle Temporal Area)/V5: perception of motion (mind that there can be a difference in [rapid vs. slow motion] perception)
V8 (formerly V4): coloured (vs. non-coloured) vision
LOC (Lateral object recognition complex): recognizing objects and faces.
How are effects of lesions in V1 and higher stages different?
Lesions in V1 lead to visual field defects, such as blind spots or blindness in the corresponding area of the visual field that the damaged portion of V1 represents. Lesions in higher stages do not lead to blindness, but rather to losing certain features (eg. perceiving of colour, motion)
