Hemodynamic changes in the infant cortex during the processing of featural and spatiotemporal information

Abstract

Over the last 20 years neuroscientists have learned a great deal about the ventral and dorsal object processing pathways in the adult brain, yet little is known about the functional development of these pathways. The present research assessed the extent to which different patterns of neural activation, as measured by changes in blood volume and oxygenation, are observed in infant visual and temporal cortex in response to events that involve processing of featural differences or spatiotemporal discontinuities. Infants aged 6.5 months were tested. Increased neural activation was observed in visual cortex in response to a featural-difference and a spatiotemporal-discontinuity event. In addition, increased neural activation was observed in temporal cortex in response to the featural-difference but not the spatiotemporal-discontinuity event. The outcome of this experiment reveals early functional specialization of temporal cortex and lays the foundation for future investigation of the maturation of object processing pathways in humans.

Figure 1

The test events seen in the Featural-Difference (A) and Spatiotemporal-Discontinuity (B) Condition. Although not pictured, a hand moved the objects. In the featural-difference condition, the ball moved right until it was fully hidden behind the occluder (2 s); the box then emerged and moved to the right edge of the platform (2 s). The box paused (1 s) and the 5 s sequence was seen in reverse. The entire 10 s ball-box cycle then repeated twice to conclude the 30 s trial. When in motion the objects moved at a rate of 12 cm/s and the occlusion interval was 1.8 s. In the spatiotemporal-discontinuity condition, the column moved right until it was fully occluded (2.5 s) and then the second column appeared immediately at the right edge of the occluder (the event was produced by two experimenters who had similar sized hands covered by identical white gloves) and moved right until it reached the right end of the platform (2.5 s). The column paused (1 s) and the 6 s sequence was reversed. The entire event was then repeated 1.5 times to conclude the 30 s trial. When visible, the object moved at a rate of 3 cm/s.

Figure 2

Location of probe placement (A) and hemodyanmic response curves (B) for each detector. Probes were placed on the skull using the International 10–20 system. One probe was positioned so that the source (S1) lay directly above the inion and the detectors (D1 and D2) lay over the right and left visual cortex, respectively. The other source (S2) was positioned at T3 and the corresponding detectors (D3 and D4) lay anterior and posterior to T3 over temporal cortex. Emitter-detector distances were 2 cm. The hemoglobin response curves, in optical density units presented as mM x cm (x-axis), were averaged across participants and trials for the four detectors. For purposes of interpretation, HbT is displayed along with HbO₂ and HbR (green, red, and blue lines respectively). The left panel of Figure 2B displays responses obtained during the featural-discontinuity event and the right panel displays responses during the spatiotemporal-discontinuity event. The event began at time 0 and continued for 30 s; 31 to 40 is post-stimulus (silent pause). The asterisks indicate points along the response curves that differed significantly from 0 (baseline).