Biomedical subjects
Publications and source records attributed to J A Mazer.
Our understanding of the neural basis of object recognition is based primarily on work with non-human primates. The problem has recently been addressed in humans using functional magnetic resonance imaging; new results indicate that the lateral occipital complex plays an important role in human object recognition.
Extrastriate area V4 is crucial for intermediate form vision and visual attention in nonhuman primates. Human neuroimaging suggests that an area in the lingual sulcus/fusiform gyrus may correspond to ventral V4 (V4v). We studied a human neurological patient, AR, with a putative V4v lesion. The lesion does not affect early visual processing (luminance, orientation, and motion perception). However, it does impair hue perception, intermediate form vision, and visual attention in the upper contralateral visual field. Form deficits occur during discrimination of illusory borders, Glass patterns, curvature, and non-Cartesian patterns. Attention deficits occur during discrimination of the relative positions of object parts, detection of low-salience targets, and orientation discrimination in the presence of distractors. This pattern of deficits is consistent with the known properties of area V4 in nonhuman primates, indicating that AR's lesion affects a cortical region functionally homologous to macaque V4.
The barn owl (Tyto alba) uses interaural time difference (ITD) cues to localize sounds in the horizontal plane. Low-order binaural auditory neurons with sharp frequency tuning act as narrow-band coincidence detectors; such neurons respond equally well to sounds with a particular ITD and its phase equivalents and are said to be phase ambiguous. Higher-order neurons with broad frequency tuning are unambiguously selective for single ITDs in response to broad-band sounds and show little or no response to phase equivalents. Selectivity for single ITDs is thought to arise from the convergence of parallel, narrow-band frequency channels that originate in the cochlea. ITD tuning to variable bandwidth stimuli was measured in higher-order neurons of the owl's inferior colliculus to examine the rules that govern the relationship between frequency channel convergence and the resolution of phase ambiguity. Ambiguity decreased as stimulus bandwidth increased, reaching a minimum at 2-3 kHz. Two independent mechanisms appear to contribute to the elimination of ambiguity: one suppressive and one facilitative. The integration of information carried by parallel, distributed processing channels is a common theme of sensory processing that spans both modality and species boundaries. The principles underlying the resolution of phase ambiguity and frequency channel convergence in the owl may have implications for other sensory systems, such as electrolocation in electric fish and the computation of binocular disparity in the avian and mammalian visual systems.
Crambin, a hydrophobic plant seed protein, consists of a single chain of 46 amino acids with a calculated molecular weight of 4720. The primary structure was determined by using solid-phase sequencing techniques and was confirmed through X-ray crystallographic analysis of the protein at 1.5-A resolution [Hendrickson, W. A., & Teeter, M. M. (1981) Nature (London) 290, 107-112]. High-performance liquid chromatographic separation of the proteolytic fragments from crambin led to the identification of two sites of microheterogeneity. The three disulfide bonds were located at positions 3-40, 4-32, and 16-26 from the crystallographic data. Comparison of the primary structure with known sequences revealed that crambin is homologous with the plant toxins purothionin and viscotoxin. Methods to estimate protein secondary structure were applied and found to predict all of crambin's structure except its amphiphilic helix.