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23/20 1. Give three functional reasons why we have so many parallel processing pathways in the visual system? One function of parallel processing in retina is to condense the transfer of information to the cortex by encoding the different qualities of an image into a set of specialized circuits, allowing the information to pass efficiently through the blind spot. These specialized circuits also function as a means of transmitting information about different aspects of the same region in space simultaneously (Nassi & Callaway, 361). Later in the visual system, existence of different parallel processing pathways allows for the information they transmit to interact if very specific ways, providing the basis for complex computations: information from one stream can be amplified while another is suppressed, or one might modulate the other, or they might be integrated into a new output stream (Nassi & Callaway, 366). 3.5/5 I count only two reasons? 2. Dorsal and ventral pathways were originally thought to serve different functional purposes, what were they? Yet, certain visual properties are processed within both visual pathways, such as binocular disparity. Why is that? Traditionally, the dorsal pathway is considered the “where” pathway: it is thought of as processing information about how objects relate to the space they are in and to the other objects in that space, carrying out motion analysis of those objects, and influencing arm and eye motor responses to them. The ventral pathway is considered the “what” pathway and is thought to be responsible for combining colors, patterns, and shapes in order to identify objects (Nassi & Callaway, 367). However, it seems that they rely on some of the same types of information to carry out their functionally different goals. One case is binocular disparity, which is the difference in information provided by each eye, and is used to derive depth information in a scene (Nassi & Callaway, 367). The dorsal pathway deals with coarse differences in binocular disparity, possibly because its work involves judging greater distances of depth (exactly how far away is that truck that’s speeding toward me?), while the ventral pathway processes fine grain differences in disparity to judge the orientation and contours of an object in order to identify it (“wait, is that actually a truck, or is it an SUV?”). 5/5 3. When there are two stimuli within a visual neuron’s receptive field, one preferred and one not preferred, how is the neural response modulated by attention? What is the evidence that there is suppression induced by attending to the non-preferred stimulus (as opposed to a relatively smaller enhancement of response when compared to attending to the preferred stimulus)? The paper considers a study that compared the effect of attending to a preferred and a nonpreferred stimulus (motion direction) in a neurons receptive field, and attending to a neutral stimulus outside of its receptive field (Treue, 296). When the animal attended to the preferred stimulus the cell became more active than when it attended to the neutral stimulus. However, when attending to the non-preferred stimulus the cell was less active then when attending to the neutral stimulus. First, the cell’s ability to respond differentially to stimuli within its 5/5 receptive field shows that attention modulation has a finer grained resolution than the size of the receptive field. Furthermore, because the response to the non-preferred stimuli fell below the response to a neutral stimulus, this shows there was active suppression. 4. In what sense does the attentional narrowing of tuning curves increase the neuron’s selectivity? In what sense does the multiplicative increase of a neurons response increase the amount of information communicated by the neuron? As a tuning curve becomes narrower the regions of steepest slope, which encode the state of the feature that the curve is tuned to respond to, cover a smaller range of values. This type of attentional modulation restricts the range of feature values that the cell responds to, though it also increases the magnitude of those responses. Thus the cell responds more selectively, while also signaling smaller variations more intensely. A multiplicative modulation also causes the regions of steepest slope to increase, but does so over the same set of feature values. Therefore, under this type of attentional manipulation, the neuron maintains the ability to communicate information about a wider range of stimulus states, while gaining the ability to signal smaller variations more intensely. 5/5 5. Attentional modulation appears to increase higher up in the visual hierarchy. Why is this (we discussed in class)? There are two possible reasons. One is that as percepts become encoded with more semantic meaning, attentional modulation becomes more behaviorally relevant, and therefore increases at higher processing areas. Another possibility is that there exists an accumulative effect of +2 attention across the visual system. When attention is “paid” at a lower level, additional resources are recruited to process a specific set of signals. The information that those signals carry is modulated to some degree as it is passed to higher levels. If those signals continue to be attended to, then additional modulation will occur at each successive stage, resulting in a compounding effect. Therefore, simply as an effect of compounding computation, attentional modulation will appear to increase as signals proceed to higher processing areas. 6. How is featural attention different than spatial attention? Why is the discovery of featural attention difficult for the “spotlight of attention” analogy? Spatial attention means boosting signals that encode information about a specific region of the visual field (i.e. something is about to appear in the left part of the scene), while featural attention boosts signals that encode information about specific properties of the visual environment (i.e. things that shift upward are about to appear in the scene). The “spotlight of attention” analogy has an implied spatial component, and refers to the way we can differentially attend to regions of the visual field. If the character of attention was entirely captured by this analogy, then only the output of the neurons that are tuned to represent the region being attended to currently would be boosted. However, even while attending to a specific region, if we are actively looking for objects that shift upward, then all neurons that are tuned to respond to upward movement for every region in the visual field will have boosted outputs (True et al., 297). You misspelled the author's name +1.5 7. How can apparently nonlinear attentional modulation of a neural response arise from multiplicative modulation in an earlier area? Multiplicative modulation at one levels means an amplification or suppression of a neuron’s output. If a neuron is tuned to respond to a preferred region, then a multiplicative modulation could cause its response to increase and decrease according to how stimuli in that region appear, but it could not, for instance, shift the boundaries of that region (True et al., 298). However, if we consider the inputs at the level below this neuron, then the receptive field of those lower level neurons tile the receptive field of the one they feed. We can imagine this region to be a circular space, filled with lots of smaller circles. In the absence of attentional modulation stimuli will elicit a response in the higher level neuron that is equal to a weighted sum of the responses at the lower level. In this case a maximum response would be caused by presenting a stimulus in the physical center of the lower level neurons combined receptive fields. However, if attention is paid to the right side of this receptive field, then the lower level neurons in that part of the region will have boosted responses. If the stimulus is shifted between the left and right sides of the receptive field under this attending condition, and we compare the response of the higher level neuron, it will respond greater when the stimulus is presented to the right side, giving the appearance that it has contracted the boundaries of its receptive field toward the right. +1