Cyclopean Discrimination Thresholds for the Direction and Speed of Motion in Depth

AbstractWe measured just-noticeable differences in the direction and speed of motion in depth of cyclopean and monocularly visible targets. Our stimulus set comprised different combinations of (dφ/dt)/(dδ/dt), dδ/dt, dφ/dt and Δδ, where dφ/dt was the angular frontal plane speed of the binocularly-fused target, dδ/dt was its rate of change of disparity and Δδ was its disparity displacement. Our three subjects based their direction discriminations entirely on the task-relevant variable (dφ/dt)/(dδ/dt), and based their speed discriminations entirely on the task-relevant variable dδ/dt. They ignored all task-irrelevant variables in both tasks. Performance on both tasks was the same for motion within the horizontal and vertical meridians. Direction discrimination threshold rose significantly as the reference direction grew more oblique with respect to a line passing midway between the eyes and perpendicular to the frontal plane. Performance on the direction discrimination task was significantly better for the noncyclopean than for the cyclopean target, but the difference was not great. For the cyclopean target, the lowest value of the direction discrimination threshold was 0.70 deg (mean of three observers and two meridians). The Weber fraction for discriminating speed was not significantly different for the cyclopean and monocularly visible targets, and did not depend on the direction of motion in depth. The lowest values (mean of three observers and two meridians) were 0.12 (cyclopean) and 0.10 (noncyclopean). Findings did not scale for viewing distance. We propose that the human visual pathway contains: (a) a cyclopean mechanism sensitive to variations in the ratio (dφ/dt)/(dδ/dt) that is comparatively insensitive to both dφ/dt and dδ/dt; and (b) a speed-sensitive cyclopean mechanism that responds to variations in the value of dδ/dt, but is comparatively insensitive to dφ/dt. We also propose that a single speed-sensitive mechanism determines speed discrimination thresholds for both cyclopean and monocularly visible targets. Copyright © 1996 Elsevier Science Ltd

Frugivoria em morcegos (Mammalia, Chiroptera) no Parque Estadual Intervales, sudeste do Brasil

<abstract language="eng">This study was carried out at the Intervales State Park, an Atlantic Rain Forest area in Southeastern Brazil. Bats were monthly mist netted over a full year, and fecal samples were collected for dietary analysis. The seeds found in each sample were identified in the laboratory under a stereoscopic microscope by comparison with seeds taken from ripe fruits collected in the study area. Three hundred and seventy one bats were collected, of which 316 (85.2%) were frugivorous. The total number of fecal samples with seeds and/or pulp was 121. Sturnira lilium (E. Geoffroy, 1810) was the most abundant species in the study area (n = 157 captures) and Solanaceae fruits accounted for 78.5% of the fecal samples with seeds (n = 56). Artibeus fimbriatus Gray, 1838 (n = 21 samples) fed mostly on Cecropiaceae (38%) and Moraceae fruits (24%), and Artibeus lituratus (Olfers, 1818) (n = 7 samples) on Cecropiaceae (57%) and Moraceae (29%). Carollia perspicillata (Linnaeus, 1758) (n = 16 samples) fed mostly on Piperaceae fruits (56,3%), but Solanaceae (31,3%) and Rosaceae seeds (12,5%) were also found in feces. Overall, seeds found in bat feces belong to eight plant families: Solanaceae (n = 67 samples); Cecropiaceae (n = 14); Piperaceae (n = 14); Moraceae (n = 8); Rosaceae (n = 3); Cucurbitaceae (n = 3); Cluseaceae (n = 1), and Araceae (n = 1). The close association of different bat species with fruits of certain plant families and genus may be related to a possible mechanism of resource partitioning that shapes the structure of the community