Week 4 Flashcards
Fovea
The area of high resolution, full colour vision, or fovea, is only 1.5 deg across, and the highest receptor density in the fovea only is 20 minutes of arc ( = 0.33 deg).
Rods
Nearly cylindrical
90-95% of human retina
greatest incidence in species that are active at night
located (or most active?)toward the periphery
no direct contribution to colour vision
strong response to dim light
little contribution to perception in detail
Cones
Tapered at one end
5-10% prevlance in human retina
Greatest incidence in birds, primates and other species that are active during the day
Area located mostly in (or is it mostly active in:?) = the fovea
CRITICAL FOR COLOUR VISION
response to dim light is weak
contribute MUCH to perception in detail
Blue Cones
Maximally response to light with wavelngth of 430nm
Rods maximally active
~ 450 to 495nm
Green cones
maximally active ~ 530nm
Red cones
maximally active ~560nm
Spectral sensitivity functions for rods and the three types of cones
Figure 5.14 Spectral sensitivity functions for rods and the three types of cones. The short-wavelength (“blue”) cones are maximally responsive to light with a wavelength of 430 nm. The peak sensitivities of the medium-wavelength (“green”) and long-wavelength (“red”) cones are shifted to longer wavelengths. White light such as daylight activates all three receptors because it contains all wavelengths.
cerebral achromatopsia
Patients with the rare condition of
cerebral achromatopsia
cannot perceive colors, due to damage of
cortex in the inferior temporal lobe of both cerebral hemispheres
Lateral Geniculate Nucleus and Cortex
The primary visual pathway in simians. This is also called the retinogeniculostriate path way
Eye/Retina > Optic Nerve > optic Chiasm > optic tract > Lateral Geniculate nucleus > optic radiations (into V1) >priimary visual cortex (v1)
another visual pathway that is used for detection of fast moving objects in the peripheral visual field
There is another visual pathway that is used for detection of fast moving objects in the peripheral visual field that drive oriented eye and head movements – it involves the superior colliculus. We will find out later that it is also involved in rapid emotional responses to visual objects.
Eye > superior colliculous > pulvinar > (amgydla) Other visual cortical areas (v1 etc)
Contralaterlism
Information from the right side of each retina goes to the right Lateral geniculate nucleus
Information from the contralateral side goes to layesr 1,4 and 6
information from the ipsilateral side goes to layers 2, 3 and 5
Layers 5 and 6 receive inpurt from the magnocellualr pathway
layers from 1-4 receive input from the parvocellular pathway
(flipped for left side of each retina?)
The two pathways
Magnoceullar path
Parvocelluarl path
The two types of pathways
convey different types of
visual information to higher
order visual areas
Summary of the prominent visual areas and the pattern of connectivity in the macaque.
Whereas all cortical processing begins in V1, there are two processing streams that extend either dorsally to the parietal lobe or ventrally to the temporal lobe. The stimulus required to produce optimal activation of a cell becomes more complex along the ventral stream. In addition, the size of the receptive fields of these cells increases, ranging from the 0.5-degree span of a V1 cell to the 40-degree span of a cell in area TEO (temporal occipital).
Summary of what the two pathways can do
Recognition: (What pway)
starts at V1 up to v2 to v3 (dynamic form) and v4 (colour form) then on to Temporal visual areas
Action (where pathway)
V1 to V5 (motion) and V3a (form) on to Parietal Visual Areas
Goal of Object Recognition
The goal of object recognition is to identify an object and determine its location.
Ungerleider and Mishkin (1982) Hypothesis
In 1982, Ungerleider and Mishkin conducted a study in which they investigated object recognition and landmark recognition. In the first task, monkeys studied an object and then, at test, had the opportunity to choose the nonstudied object to receive a reward. In the landmark task, monkeys learned about landmarks and then were asked to choose the foodwell that was closest to a tall cylinder (the landmark) for a reward. The researchers found that temporal lesions impaired the object task whereas parietal lesions resulted in impaired performance on the landmark task. Thus, these focal lesions resulted in selective deficits and provided evidence for a double dissociation.
The What (Ventral) Pathway
V1 > inferior longitudinal fasiculous > inferior temporal cortex
WHAT pathway = temporal
Object perception and recognition
The where pathyway )(dorsal stream)
v1 > Superior longitudinal fasiculus > posteroparietal cortex
Where pathway = Dorsal = parietal
Spatial perception
Integration of the pathways is thought to occur
in the striate cortex
How Features are analysed
Hubel and Wiesel’s discovery of motion detecting cells and complex cell
Complex cell receptive field strong response when stimulus is at a 45degree angle and moving
Activation of MT neurons influences the perceived direction of motion.
Trial sequence. Two red dots indicate possible directions of motion (up and to the right or downward). In 50% of the trials, electrical stimulation was briefly applied in area when the stimulus was presented. The stimulation was directed at neurons with a known preferred direction. After the stimulus, the monkey looks at one of the two red dots to indicate the perceived direction of motion. (b) When the stimulus was present, the monkey was more likely to respond that the direction of motion was in the direction of the preferred direction of the electrically stimulated cells. The x-axis indicates the strength of the motion signal, with 0% indicating random motion, negative values indicating motion in the opposite direction of the cell’s preferred direction, and positive values, motion in the direction of the cell’s preferred direction.
Larger values mean more of the dots moved in the indicated direction.
Single-cell recordings from a neuron in the inferior temporal cortex – respond to particular shape
These cells rarely respond to simple stimuli such as lines or spots of light. Rather, they respond to more complex objects, such as the hand drawings shown in the first five images. Note that the cell shows only a weak response to the mitten, indicating that its activity is not associated with the general shape of a hand. This cell does not respond to the comb like objects that contain the series of parallel lines (images 7, 9, and 10) as in the hand stimuli.
Matching task for dissociation of position and object discrimination
Object and position matching to sample task. The Study and Test displays each contain three objects in three positions. On object retrieval trials, the participant judges if the three objects were the same or different. On position retrieval trials, the participant judges if the three objects are in the same or different locations. In the examples depicted, the correct response would be “same” for the object task trial and “different” for the position task trial. Left – note different activations for each task.
Patient D.F.
Patient D.F. – damage to the ventral stream – lateral occipital complex (LOC)
Patient D.F. – damage to the ventral stream – lateral occipital complex (LOC)
(a) The patient performed poorly in the explicit matching task when asked to match the orientation of the card to that of the slot. (b) In the action task, the patient was instructed to insert the card in the slot. Here, she produced the correct action without hesitation.
The key here is accessibility of information. When explicitly matching the orientation, D.F’.’s visual agnosia hampered her ability to use the object’s orientation information to complete the task. However, when asked to insert the card, D.F. had access to the spatial information through her dorsal “where” pathway, which guided her interaction with the card.
Structural MRI scans showed that D.F. has widespread cortical atrophy with concentrated bilateral lesions in the ventral stream that encompass the lateral occipital cortex (LOC) (Figure 6.6; T. James et al., 2003).
Computational Problems in Object Recognition
Analyzing shape and form.
Despite the irregularities in how these objects are depicted, we have little problem recognizing them. We may never have seen pink elephants or plaid apples, but our object recognition system can still discern the essential features that identify these objects as elephants and apples.
Low level to high level
eg 3 straight lines = same low level components
become a triangele or an arrow (two different percepts)
Component analysis of object recognition
Stimuli for the three conditions and the mental operations required in each condition. Novel objects are hypothesized to engage processes involved in perception even when verbal labels do not exist.
in familiar stimuli > feature extraction, shape description and memory mathcing all necessary
novel > memory is not necessary
scrambled only feature extraction
Activation was greater for the familiar and novel objects compared to the scrambled images along the ventral surface of the occipitotemporal cortex.
Hierarchical coding hypothesis
Elementary features are combined to create objects that can be recognized by gnostic units. At the lowest level of the hierarchy are edge detectors, units that operate similar to the way the simple cells discussed in Chapter 5 operate. These feature units combine to form corner detectors, which in turn combine to form cells that respond to even more complex stimuli, such as surfaces. (a) The hypothesized computational stages for hierarchical coding. (b) A neural implementation of the computational stages illustrated in part (a), based on the idea of grandmother cells
Recognising Faces
Identifying face cells in the superior temporal sulcus of the macaque monkey. The graphs in (b) show the responses of two cells to the 10 stimuli (labeled A–J) in (a). Both cells responded vigorously to many of the facial stimuli. Either there was no change in activity when the animal looked at the objects, or, in some cases, the cells were actually inhibited relative to baseline. The firing-rate data are plotted as a change from baseline activity for that cell when no stimulus was presented.
Isolating neural regions during face perception.
Bilateral activation in the fusiform gyrus was observed with fMRI when subjects viewed collages of faces and random patterns compared with collages of just the random patterns. Note that, following neuroradiological conventions, the right hemisphere is on the left.
Beyond One Sense – Multimodal Integration
Visual, auditory, and somatosensory spatial representations in the superior colliculus.
The overlapping of the common coordinate systems can lead to a representation of multisensory space.
McGurk Effect
In the McGurk effect, vision modulates auditory perception (Alsius et al., 2005; McGurk and MacDonald, 1976). Although there are many example of crossmodal attention, in the McGurk effect, perception of phonemes is affected by simultaneous visual information concerning the speaker’s facial movements.
Multimodal Perception
(a) Multisensory integration occurs subcortically (e.g., thalamus). Input to cortical areas is already influenced by information from other sensory modalities. (b) Modality-specific regions are surrounded by multisensory regions that receive input from other modalities. (c) Multisensory interactions occur through communication between modality specific regions. (d) Certain cortical areas are specialized for multisensory processing. PP = posterior parietal cortex; STS = superior temporal sulcus.
The neural basis of visual illusions
(a) When viewing the Enigma pattern, we perceive illusory motion. Viewing the pattern is accompanied by activation in area MT. (b) Staring at the bright green patch produces color aftereffects. (c) Activation in a visual region anterior to V4 is plotted (see text for details).
