To gain insight into how cortical fields process somatic inputs and ultimately contribute to complex abilities such as tactile object perception, we examined the pattern of connections of two areas in the lateral sulcus of macaque monkeys: the second somatosensory area (S2), and the parietal ventral area (PV). Neuroanatomical tracers were injected into electrophysiologically and/or architectonically defined locations, and labeled cell bodies were identified in cortex ipsilateral and contralateral to the injection site. Transported tracer was related to architectonically defined boundaries so that the full complement of connections of S2 and PV could be appreciated. Our results indicate that S2 is densely interconnected with the primary somatosensory area (3b), PV, and area 7b of the ipsilateral hemisphere, and with S2, 7b, and 3b in the opposite hemisphere. PV is interconnected with areas 3b and 7b, with the parietal rostroventral area, premotor cortex, posterior parietal cortex, and with the medial auditory belt areas. Contralateral connections were restricted to PV in the opposite hemisphere. These data indicate that S2 and PV have unique and overlapping patterns of connections, and that they comprise part of a network that processes both cutaneous and proprioceptive inputs necessary for tactile discrimination and recognition. Although more data are needed, these patterns of interconnections of cortical fields and thalamic nuclei suggest that the somatosensory system may not be segregated into two separate streams of information processing, as has been hypothesized for the visual system. Rather, some fields may be involved in a variety of functions that require motor and sensory integration.

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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...