Cortical activation during color discrimination task in macaques as revealed by positron emission tomography

Parallel, multi-stage processing of colors, faces and shapes in macaque inferior temporal cortex

Visual-object processing culminates in inferior temporal cortex (IT). To assess the organization of IT, we measured functional magnetic resonance imaging responses in alert monkeys to achromatic images (faces, fruit, bodies and places) and colored gratings. IT contained multiple color-biased regions, which were typically ventral to face patches and yoked to them, spaced regularly at four locations predicted by known anatomy. Color and face selectivity increased for more anterior regions, indicative of a broad hierarchical arrangement. Responses to non-face shapes were found across IT, but were stronger outside color-biased regions and face patches, consistent with multiple parallel streams. IT also contained multiple coarse eccentricity maps: face patches overlapped central representations, color-biased regions spanned mid-peripheral representations and place-biased regions overlapped peripheral representations. These results show that IT comprises parallel, multi-stage processing networks subject to one organizing principle.

Color signals through dorsal and ventral visual pathways

Explanations for color phenomena are often sought in the retina, lateral geniculate nucleus, and V1, yet it is becoming increasingly clear that a complete account will take us further along the visual-processing pathway. Working out which areas are involved is not trivial. Responses to S-cone activation are often assumed to indicate that an area or neuron is involved in color perception. However, work tracing S-cone signals into extrastriate cortex has challenged this assumption: S-cone responses have been found in brain regions, such as the middle temporal (MT) motion area, not thought to play a major role in color perception. Here, we review the processing of S-cone signals across cortex and present original data on S-cone responses measured with fMRI in alert macaque, focusing on one area in which S-cone signals seem likely to contribute to color (V4/posterior inferior temporal cortex) and on one area in which S signals are unlikely to play a role in color (MT). We advance a hypothesis that the S-cone signals in color-computing areas are required to achieve a balanced neural representation of perceptual color space, whereas those in noncolor-areas provide a cue to illumination (not luminance) and confer sensitivity to the chromatic contrast generated by natural daylight (shadows, illuminated by ambient sky, surrounded by direct sunlight). This sensitivity would facilitate the extraction of shape-from-shadow signals to benefit global scene analysis and motion perception.

Molecular imaging of conscious, unrestrained mice with AwakeSPECT

We have developed a SPECT imaging system, AwakeSPECT, to enable molecular brain imaging of untrained mice that are conscious, unanesthetized, and unrestrained. We accomplished this with head tracking and motion correction techniques.

METHODS

The capability of the system for motion-corrected imaging was demonstrated with a (99m)Tc-pertechnetate phantom, (99m)Tc-methylene diphosphonate bone imaging, and measurement of the binding potential of the dopamine transporter radioligand (123)I-ioflupane in mouse brain in the awake and anesthetized (isoflurane) states. Stress induced by imaging in the awake state was assessed through measurement of plasma corticosterone levels.

RESULTS

AwakeSPECT provided high-resolution bone images reminiscent of those obtained from CT. The binding potential of (123)I-ioflupane in the awake state was on the order of 50% of that obtained with the animal under anesthesia, consistent with previous studies in nonhuman primates. Levels of stress induced were on the order of those seen in other behavioral tasks and imaging studies of awake animals.

CONCLUSION

These results demonstrate the feasibility of SPECT molecular brain imaging of mice in the conscious, unrestrained state and demonstrate the effects of isoflurane anesthesia on radiotracer uptake.

In vivo imaging of epileptic activity using 2-NBDG, a fluorescent deoxyglucose analog

Accurately locating epileptic foci has great importance in advancing the treatment of epilepsy. In this study, epileptic seizures were first induced by intracortical injection of 4-aminopyridine in rats. A fluorescent deoxyglucose substitute, 2-(N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)amino)-2-deoxyglucose (2-NBDG), was then continuously injected via the tail vein. Brain glucose metabolism was subsequently monitored by fluorescence imaging of 2-NBDG. The initial uptake rate of 2-NBDG at the injection site of 4-aminopyridine significantly exceeded that of the control injection site, which indicated local hypermetabolism induced by seizures. Our results show that 2-NBDG can be used for localizing epileptic foci.