RAPID COMMUNICATION Temporal Modulation of Spatial Borders in Rat Barrel Cortex

Sheth, Bhavin R., Christopher I. Moore, and Mriganka Sur. sponses might be the significant differences in response charTemporal modulation of spatial borders in rat barrel cortex. J. acteristics of the two thalamic nuclei ( the ventral posterior Neurophysiol . 79: 464–470, 1998. We examined the effects of medial (VPm) nucleus and the posterior medial (POm) nuvarying vibrissa stimulation frequency on intrinsic signal and neucleus) that project to rat barrel cortex and their anatomic ronal responses in rat barrel cortex. Optical imaging of intrinsic projections to PMBS (see DISCUSSION). The anatomic and signals demonstrated that the region of cortex activated by deflecphysiological segregation of these two thalamocortical pathtion of a single vibrissa at 1 Hz is more diffuse and more wideways suggests that the input to regions surrounding the barspread than the territory activated at 5 or 10 Hz. With the use of rels in layer IV might be attenuated at higher frequencies two different paradigms, constant time of stimulation and constant and hence that there might be a corresponding drop-off in number of vibrissa deflections, we showed that the optically imaged spread of activity is more discrete at higher stimulation frequencies. the strength of optical signal in these surrounding regions. We combined optical imaging with multiple electrode recording Finally, it is important to define stimulus parameters for and confirmed that the neuronal response to individual vibrissa PMBS cortex that activate either 1) predominantly a barrel / stimulation at the optically imaged center of activity is greater than column by evoking a spatially discrete and localized cortical the response away from the imaged center. Consistent with the response or 2) neurons in interbarrel regions and in other imaging data, these recordings also showed no response to a second neighboring barrels as well by evoking a more spatially difvibrissa deflection at 5 Hz at a peripheral recording site, though fuse cortical response. Knowledge of stimulus parameters, there was a significant response to a second vibrissa deflection at such as frequency of vibrissa stimulation (Blood et al. 1 Hz at the same peripheral site. These findings demonstrate that 1995), that elicit a robust and reproducible optical response vibrissa stimulation at higher frequencies leads to more focused in rat barrel cortex is of obvious importance for future studies physiological responses in cortex. Thus the spread of activation in rat barrel cortex is modulated in a dynamic fashion by the freof this system. quency of vibrissa stimulation. To investigate these issues, we used intrinsic signal optical imaging (Grinvald et al. 1986; Masino and Frostig 1996; Masino et al. 1993; Narayan et al. 1994) in rat barrel cortex I N T R O D U C T I O N to study the effects of stimulus frequency on cortical response. Data from both somatosensory (Masino et al. 1993) Layer IV of rodent somatosensory cortex contains discrete clusters of cells commonly referred to as ‘‘barrels’’ and visual (Das and Gilbert 1995; Toth et al. 1996) cortices suggest that the optical signal correlates with the activity (Woolsey and Van der Loos 1970). Within the posterior medial barrel subfield (PMBS) of rat somatosensory cortex, (extracellular plus intracellular; Moore et al. 1996) of local neuronal populations. We also correlated the optically imneurons in a given barrel column are most responsive to deflection of a single vibrissa on the rat’s snout, although aged map with simultaneous recording of neuronal responses at multiple sites in the map. neurons in each barrel are also responsive to stimulation of other vibrissae (Armstrong-James and Fox 1987; Simons 1978, 1985), with neurons in neighboring barrels maximally M E T H O D S responsive to adjacent vibrissae on the rat’s snout. This soSprague-Dawley rats (8 animals) ranging in weight from 175 matotopy, revealed by conventional electrophysiological to 325 g were anesthetized with urethan (1.25 g/kg ip) and the techniques (Welker 1971, 1976), renders optical imaging an depth of anesthesia was continually monitored so that the animal ideal tool for investigating questions regarding the functional remained areflexive to hindpaw pinch and the breathing rate reproperties of neuronal ensembles in rat barrel cortex. This mained between 80–120 breaths/min (Armstrong-James and Fox technique allows us to observe metabolic activity over a 1987). If the hindpaw reflex was observed or if the breathing rate large expanse of cortical territory (several millimeters enexceeded 120 breaths/min, a supplemental dose of urethan (10% compassing PMBS) and to thereby observe changes in the ip initial dose) was delivered. spatial pattern of functional activity of a population of neurons. Stimulation Recent studies have shown that rats whisk their vibrissae The optical response to displacement of individual vibrissae at at a dominant frequency of about 8 Hz (Carvell and Simons different frequencies was measured. The vibrissa stimulator con1990). Similarly other studies have found that rats exploring sisted of a glass capillary attached to a piezoelectric ceramic wafer their environment sweep their vibrissae at 6–9 cycles/s. (1.5 1 0.5 in., Piezo Systems 2). The vibrissae on the left side of (Welker 1964; see also Wineski 1983; Woolsey et al. 1981). the rat’s face were clipped to a length of 1 cm. The tip of the glass An important question is whether or not and how the pattern capillary was then placed either over the shaft of the vibrissa or of cortical activity changes with vibrissa movement in this beneath it, so that the point of deflection was 5 mm from the range of behaviorally relevant frequencies. base of the vibrissa (Armstrong-James et al. 1994). A computercontrolled pulse triggered a stimulus generator to send a voltage A possible basis for frequency-dependent cortical re-