Tom Parry 1 Q551 neural correlates of Phenomenological experience assignment Are the same neural correlates involved in the perception of static and dynamic multistable percepts: An fMRI analysis Multistable percepts have long been studied to facilitate the understanding of the visual system. The benefit of using multistable percepts to analyze visual awareness is the spontaneous reversals of stimulus interpretation they evoke, allowing the dissociation of perceptual-attention neural mechanisms from stimulus-driven neural mechanisms. Many different types of stimuli have been used from the necker cube, a static stimulus where depth perception of the cube alternates, to oscillating dots, a more dynamic stimuli where perceived motion is the alternating percept. The use of different visual stimuli has brought about various different interpretations of results and therefore identification of the visual areas involved has also varied. However, it has been suggested that although both types of stimuli appear to excite similar visual areas in the brain, they are essentially different and should be analyzed separately. Early work by Ellis, Wong & Stark (1979) suggested perceptual reversals may be a function of axial eye movements which alter the image transposed onto the retina, and therefore alters the perception of the stimuli. However, when eye movements have been controlled reversals both exogenously and endogenously, have still occurred. Yet, this research does bring about an important question related the control of perceptual reversals. Endogenous reversals have been shown to occur at relatively predictable rates per individual, but have not been under the direct regulation of these subjects as to when they occur. Van Ee, van Dam & Brouwer (2005) investigated this control of perceptual reversals and demonstrated supporting results. They instructed subjects to either hold aTom Parry 2 particular interpretation or speed-up the reversal process, i.e. switch on command. The results illustrated that subjects could not control the specific time frame of reversal, as they appeared to happen as spontaneously as non-instructed subjects. This provides support for a threshold mechanism may be in place which dictates when the reversals take place. Gaetz, Weinberg, Rzempoluck & Jantzen (1998) provide further support for this idea of endogenous control. Using necker cube stimuli, they acquired data using MEG and EEG recording over the occipital, parietal and temporal lobes and found correlated cortical activity. In conclusion they suggested a complex integration of visual, memory and attention mechanisms may be related to the bistable nature of necker cube reversals. This explanation has been echoed throughout the literature for static stimuli such as the necker cube, but exogenous reversals are difficult to elicit using this type of stimulus. Although this research can clearly demonstrate the possibility of executive mechanisms for endogenous reversals in static stimuli, little work has focused on the comparison of both endogenously and exogenously controlled reversals, and the source of their control mechanisms. Kornmeier & Bach (2005) identify two explanations of bottom-up or top-down influences in early visual processing, however, the role of visual awareness remains unclear, as they suggest that many reversals may occur before P300, prior to visual awareness being established. The importance of different cortical areas in the perception of ambiguous stimuli has implicated activity in the sensory visual area. Leopold & Logothetis (1999) suggest that not only is this area essential in ambiguous perception, but also supports the role of many top-down influences and extrastriate areas in ambiguous vision. Using fMRI theyTom Parry 3 also demonstrated frontal and parietal areas associated with cognitive behaviors also appear to be activated, implying the possible influence of awareness. Another fMRI study by Inui, Tanaka, Okada, Mishizawa, Katayama & Konishi (2000) demonstrated cortical network connections between spatial attention and the motor execution system. They suggest visual cues are mapped directly to parameters which are relevant to motor function and hence, underly the importance of response preparation in the perception of visual stimuli. Slotnick & Yantis (2005) further support the idea of spatial attention as a mediator of perceptual reversals from fMRI data. The data showed activation of the posterior parietal cortex during voluntary shifts in perceptual configuration, suggesting the involvement of attentional mechanisms. Many of the above studies have used necker cube reversals as the imperative stimuli. However, little has been compiled on the neural correlates of dynamic stimuli and whether they differ from such static stimuli. Struber & Herrmann (2002) utilized the oscillating dots dynamic stimulus to investigate visual awareness using MEG measurements. They demonstrated a destabilization process until the reversal occurred which was linked to a switch of visual awareness. It was also noted that subjects were consciously aware of pattern reversals, implicating awareness as an essential component of the reversal. The purpose of the present study is to identify the role of the visual cortex heiracrchy and attentional processes in the perception of both endogenous and exogenous perceptual reversals. fMRI will be utilized to gain a greater spatial resolution of the cortical areas involved.Tom Parry 4 Methods Subjects 20 subjects will be used for this study, with ten male and 10 female subjects being recruited. All subjects will be right handed and will have normal or corrected vision. Stimuli The stimuli to be used will be similar to those used by Struber & Herrmann (2002) with a series of oscillating dots to induce ambiguous, horizontal or vertical apparent motion (See figure 1). Figure 1. Stimuli Presentation of A1 and A2 resulted in the perception of apparent motion in either the horizontal or vertical direction, which was the main manipulation of the current experiment. The presentation of HL-HR and VT-VB resulted in the perception of horizontal or vertical motion respectively. The alternation of A1 and A2 will induce spontaneous endogenous reversals as the percept will be under the control of internal switches of attention. The rapid alternation of the HL-HR and VT-VB patterns will trigger an exogenous reversal of apparent motion due to the clear horizontal to