IUB COGS-Q 551 - Exploring neural correlates of motion-induced blindness through zapping visual cortex

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Exploring neural correlates of motion-induced blindness through zapping visual cortex By Krystal A. Klein, Angela B. Nelson, & Eric A. Dimperio Department of Psychological and Brain Sciences: Indiana University 1. Objectives Motion-induced blindness (MIB) refers to a phenomena where one can fail to perceive a salient object within their visual field. MIB is not explained by current theories of visual processing and could provide important constraints upon these models. We will utilize transcranial magnetic stimulation (TMS) to induce transient alterations to normal neuronal activity, allowing us to examine the role of specific visual areas thought to be critical to producing MIB. We plan to examine the effects of TMS to two cortical sites: V5 and V1. We will also include a condition when stimulation to V5 is followed by stimulation to V1, because the V5-V1 feedback has been shown to be critical in perception of motion (Pascual-Leone & Walsh, 2001). We hope to produce results that will give us further insight into the specific roles these areas and the connections between them play in the phenomenon of MIB. 2. Background and Significance 2.1 Visual Disappearance Phenomena A popular field in psychology today is the study of a class of stimuli known as visual disappearance phenomena, where under normal (monocular) viewing conditions; stimuli with unchanging salience cease to be perceived. A well-known example of this type of occurrence is binocular rivalry, where two different images are presented to opposite eyes and the images are alternately perceived. Another less well studied example is the phenomenon of motion-induced blindness. They still remain almost completely unexplained even though they can be verydramatic to experience. The most dramatic can be cases of binocular rivalry and motion-induced blindness, where perception switched back and forth in a predictable cycle. Explanations of binocular rivalry center on the idea of conflicting images. However, in motion-induced blindness, there is no obvious conflict. Bonneh, Cooperman, and Sagi (2001) have even demonstrated the phenomena when the items that disappear and reappear have a zone of protection around them where no moving particles are allowed. It seems as if motion in the periphery can induce a stationary object in another area of the periphery to disappear. 2.2 Transcranial Magnetic Stimulation The recent advent of transcranial magnetic stimulation technology has provided researchers with a powerful but noninvasive new tool with which to observe and characterize neural function and connectivity (see Robertson, Theoret, & Pascal-Leone, 2003, for a review). In TMS, a powerful magnetic field is induced using a wire coil placed against a patient’s scalp; this magnetic field causes electrical stimulation in a small area of the underlying cortical tissue. These pulses are thought to decrease the signal to noise ratio of the normal brain activity in the affected region, causing the degradation of the information that should be represented at that point in the pathways of processing; this often causes an observable change of behavior; in a sense, TMS is comparable to causing a temporary and localized cortical lesion. TMS studies are advantageous over lesion studies because a relatively controlled cortical region can be selected, allowing better localization of function, and also because lesion patients are a limited resource. TMS is advantageous over neuroimaging because it is an experimental method, and therefore can be used to infer causation, while neuroimaging studies are merely correlative. 2.3 TMS Studies of Motion PerceptionIn motion-induced blindness, stationary targets surrounded by a coherently moving cloud of dots disappear and reappear in a cyclic fashion. Some researchers have attributed MIB to hemispheric differences, based on clinical cases suggesting the involvement of non-sensory attentional mechanisms or gating mechanisms: Funk and Pettigrew (2003) found that TMS of the left hemisphere produced greater disruption the cyclic appearance and disappearance of the stationary dots than did right hemisphere stimulation, Given these results, Funk and Pettigrew acknowledge that since the left hemisphere has been shown to be preferential to motion processing, the disruption of MIB could be due to specific disruption of motion processing. They also suggest that interfering with MT in particular, which has been shown to play a large role in three-dimensional motion processing, should disrupt the perception of the moving sphere. The areas of V5 and MT are important not only for the analysis of movement, but also for attention to movement. Since one possible account for MIB involves attentional mechanisms, it follows that the role of V5 should be investigated. However, studies of other visual disappearance phenomena have shown that critical neuronal events for the perceptual switches can also occur in earlier stages of processing. Using fMRI, Polonsky, Blake, Braun, and Heeger (2000) found that neural processing critical for binocular rivalry is expressed as early as V1. In addition to characterizing the independent roles of V1 and MT+/V5 in motion processing, some researchers have used TMS methods to observe the connections between these areas. Pascual-Leone and Walsh (2001) used TMS at MT+/V5 to induce moving phosphemes, and paired this stimulation with stimulation to V1 either before, at the same time as, or after the MT+/V5 stimulation. They found that stimulation to V1 before or at the same time as MT+/V5 prevented appearance of moving phosphemes normally caused by MT+/V5 stimulation, but stimulation 5 to 45 ms. following MT+/V5 stimulation caused phosphemes to freeze. TheFigure 1: Sample stimulus with stationary yellow discs and blue dots in motion. authors interpreted these findings as reflecting the importance of fast backprojections from MT+/V5 to V1 in motion perception. 3. Research Methods 3.1 Visual Stimuli The stimulus we intend to use is a modification of the one used in studies by Bonneh et al. (2001) and Funk and Pettigrew (2003) found at www.uq.edu.au/nuq/jack/rivalry.html. That stimulus consists of three stationary yellow discs arranged in an equilateral triangle. The three discs are surrounded by a collection of randomly spaced blue spots that move as if they were on the surface of a sphere rotating at a constant velocity. We intend to have only two yellow discs arranged


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IUB COGS-Q 551 - Exploring neural correlates of motion-induced blindness through zapping visual cortex

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