The perception of the spatial location of an auditory stimulus can be captured by a spatially disparate visual stimulus, a phenomenon known as the ventriloquism effect. This study investigated the temporal and spatial dependency of this illusion. In the temporal domain, only disparities of 50-100 ms were perceived as simultaneous, and disparities where the visual stimulus occurred before the auditory stimulus were more effective in creating the illusion. In the spatial domain, the illusion was elicited most strongly at spatial disparities below spatial discrimination thresholds. There was also a significant interaction between temporal and spatial disparities. These results indicate that both temporal and spatial parameters are critical in the perception of real world objects in extrapersonal space.

How well does the functional MRI (fMRI) signal reflect underlying electrophysiology? Despite the ubiquity of the technique, this question has yet to be adequately answered. Therefore, we have compared cortical maps generated based on the indirect blood oxygenation level-dependent signal of fMRI with maps from microelectrode recording techniques, which directly measure neural activity. Identical somatosensory stimuli were used in both sets of experiments in the same anesthetized macaque monkeys. Our results demonstrate that fMRI can be used to determine the topographic organization of cortical fields with 55% concordance to electrophysiological maps. The variance in the location of fMRI activation was greatest in the plane perpendicular to local vessels. An appreciation of the limitations of fMRI improves our ability to use it effectively to study cortical organization. U ntil recently, noninvasive techniques used to image the human brain and its activity were not widely accessible. However, in the past few years, procedures such as functional MRI (fMRI) have become readily available and are used in a wide range of disciplines including Radiology, Psychology, Psychiatry, Neurology, Neurosurgery, and Neuroscience. The investigations undertaken by different groups are diverse and range in scope from understanding complex perceptual and cognitive processes to examining the activity patterns generated from simple sensory stimuli. However, a basic question that has yet to be addressed is: how accurately do changes in the vascular system reflect changes in neural activity (1)?Despite the ubiquity of the technique and the number of inferences that have been made based on its use, there have been few studies that focus on the relationship between the blood oxygenation level-dependent (BOLD; ref.2) signal of fMRI and the underlying neural activity that it is assumed to reflect. Typically, fMRI is based on the hemodynamic response to evoked neural activity. The BOLD signal is derived from the oxygenation state of local hemoglobin. Neural activity requires oxygen, which triggers an increase in local oxy-hemoglobin, resulting in an increase in signal intensity in an active region of cortex (3). Thus, the BOLD signal is an indirect indicator of neural activity with unknown accuracy.Appreciating the link between fMRI and neurophysiology is important not only for the use and interpretation of fMRI, but also when comparing data from humans with the wealth of data gathered using invasive techniques in monkey cortex. The macaque monkey is the most widely studied animal model for human neocortical organization and function. The ability to compare human data directly with an animal model will allow us to extend our understanding of complex human behavior based on detailed neurophysiological and neuroanatomical data from the macaque monkey.We have, therefore, developed a monkey model appropriate for the study of the relationship between the fMRI BOLD signal and the underlying neural activity. We have examined the BOLD response and th...

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The dorsal ventricular ridge (DVR) of reptiles is one of two regions of the reptilian telencephalon that receives input from the dorsal thalamus. Although studies demonstrate that two visual thalamic nuclei, the dorsal lateral geniculate and rotundus, send afferents to the dorsal cortex and DVR, respectively, relatively little is known about physiologic representations. The present study determined the organization of the visual recipient region of the iguana DVR. Microelectrode mapping techniques were used to determine the extent, number of subdivisions, and retinotopy within the visually responsive region of the anterior DVR (ADVR). Visually responsive neurons were restricted to the anterior two thirds of the ADVR. Within this region, two topographically organized subdivisions were determined. Each subdivision contained a full representation of the visual field and could be distinguished from the other by differences in receptive field properties and reversals in receptive field progressions across their mutual border. A third subdivision of the ADVR, in which neurons are responsive to visual stimulation is also described; however, a distinct visuotopic representation could not be determined for this region. This third region forms a shell surrounding the lateral, dorsal, and medial aspects of the topographically organized subdivisions. These results demonstrate that there are multiple physiologic subdivisions in the thalamic recipient zone of the ADVR of the iguana. Comparisons to the ADVR of other reptiles are made, homologies to ectostriatial regions of the bird are proposed, and the findings are discussed in relation to telencephalic organization of other vertebrates.