We measured changes in the sensitivity of the human rod pathway by testing visual reaction times ... more We measured changes in the sensitivity of the human rod pathway by testing visual reaction times before and after light adaptation. We targeted a specific range of conditioning light intensities to see if a physiological adaptation recently discovered in mouse rods is observable at the perceptual level in humans. We also measured the noise spectrum of single mouse rods due to the importance of the signal-to-noise ratio in rod to rod bipolar cell signal transfer. Using the well-defined relationship between stimulus intensity and reaction time (Piéron's law), we measured the reaction times of eight human subjects (ages 24-66) to scotopic test flashes of a single intensity before and after the presentation of a 3-minute background. We also made recordings from single mouse rods and processed the cellular noise spectrum before and after similar conditioning exposures. Subject reaction times to a fixed-strength stimulus were fastest 5 seconds after conditioning background exposure (7...
CFF thresholds were determined for 3 normal and 3 binocularly deprived (BD) cats utilizing a simu... more CFF thresholds were determined for 3 normal and 3 binocularly deprived (BD) cats utilizing a simultaneous two-choice discrimination procedure. For all cats when adaptation level was steady-state for stimulus luminance. the CFF/luminana: function showed a rod-cone break which took the form of a mesopic null. When the cats were maintained in a light-adapted state CFF declined as a monotonic function of luminance. Under most luminance and adaptation levels the BD cats' CFF was slower than that of the normal cats. The BD cats' depressed CFF is interpreted as a result of their LGNd Y-cell loss.
Adult cats were trained to discriminate blue from green and gray. Although the cats could discrim... more Adult cats were trained to discriminate blue from green and gray. Although the cats could discriminate the intensity of stimuli whose areas ranged from 33 to 0.36 square centimeters they could not discriminate color when the stimulus was 0.36 square centimeter (less than 20 degrees visual angle). This influence of stimulus size may account for both positive and negative results of previous studies.
Our goal was to investigate the effect of displayed image magnification on perception of the size... more Our goal was to investigate the effect of displayed image magnification on perception of the size of hepatic lesions on abdominal computed tomography (CT) scans. Institutional review board approval and informed observer consent were obtained. Three experienced radiologists reviewed 90 CT image pairs in one session. Each image pair demonstrated a solitary, welldefined hypodense hepatic lesion measuring greater than 1 cm obtained at two points in time. The image pairs were presented three times in random order, once with the left image magnified, once with the right image magnified, and once with neither image magnified. The radiologists were asked to determine on which image the lesion was smaller or if there was no difference. The responses were analyzed statistically. The proportion of correct responses increased significantly as the difference in lesion size increased (p< 0.001). The percent of correct responses was higher when neither CT image was magnified. Magnification of one image decreased the accuracy of the readers' performance, especially at smaller differences, both of which were statistically significant (p<0.001). Thus, accuracy of detect-ing lesion size differences was degraded when the images were presented at differing magnification. This should be kept in mind when evaluating serial CT scans for growth or regression of tumors and other lesions.
The visual acuity of seven midland banded water snakes was measured by recording evoked responses... more The visual acuity of seven midland banded water snakes was measured by recording evoked responses from telencephalon to temporally modulated square wave grating patterns. Using conventional electrophysiological techniques and signal averaging, high contrast square wave gratings of diVerent spatial frequencies were presented. Acuity was estimated by extrapolating relative response amplitude/log 10 spatial frequency functions which yielded an average acuity of 4.25 cycles/degree. Refractive state was also estimated by recording evoked potentials to intermediate spatial frequencies with diVerent lenses in front of the eye. Polynomial Wts indicated that under the experimental conditions the snakes were around 6.4 diopters hyperopic suggesting a corrected acuity of 4.89 cycles/ degree. Reduction of grating luminance resulted in a reduction in evoked potential acuity measurements. These results indicate that the spatial resolution of midland banded water snakes is the equal of cat; about 20/120 in human clinical terms.
Four cats were raised with binocular eyelid suture and, after their eyes were opened, were traine... more Four cats were raised with binocular eyelid suture and, after their eyes were opened, were trained on a series of discrimination tasks. They performed a t normal rates on the brightness task but indicated some difficulty with the pattern tasks. They then received large, bilateral occipito-temporal cortex ablations. Postoperatively, this in no observable way affected their visually guided orienting behavior, but it did destroy their capacity to perform the preoperatively learned pattern tasks. Postoperative performance on the brightness task remained good. These data indicate that, in these deprived cats, there is little or no cortical development for visual orienting, but cortex is necessary for visual discrimination learning.
Critical flicker fusion was determined over a wide luminance range for six Siamese and four norma... more Critical flicker fusion was determined over a wide luminance range for six Siamese and four normal cats. The Siamese cats had the lowest CFF at all luminance levels when compared to normal and binocularly deprived (BD) cats. The Siamese cat's inferior temporal resolution is most likely due to their profound y-cell loss because (1) CFF is proportional to y-cell population across normal, BD, and Siamese cats; and (2) unlike BD cats, the visual cortex of Siamese cats is comparatively normal. The implication of this finding for the CFF of dark-reared cats and normal cats viewing a stimulus which does not stimulate y-cell is discussed.
Thirteen-lined ground squirrels (Citellus tridecemlineatus) were trained on three two-choice visu... more Thirteen-lined ground squirrels (Citellus tridecemlineatus) were trained on three two-choice visual discrimination problems: light/dark, color and stripe orientation. After posterior neocortical lesions in one or two stages, they were tested on all three discriminations. The results demonstrate that animals with large posterior neocortical lesions which produced retrograde changes throughout the dorsal lateral geniculate (LGNd) were capable of light/dark and wavelength discrimination. These animals were not able to discriminate stripe orientation. It is proposed that wavelength discrimination depends on extrageniculostriate mechanisms in posterior neodecorticates of this species.
We were able to train cats raised with sutured eyelids to perform simple brightness discriminatio... more We were able to train cats raised with sutured eyelids to perform simple brightness discriminations before their lids were parted. If, and only if, a small hole was present in a lid, could some of the cats also perform a grating orientation discrimination.
Visual acuity was determined for both eyes of long-term monocular-deprived cats over a wide lumin... more Visual acuity was determined for both eyes of long-term monocular-deprived cats over a wide luminance range. The influence of luminance on the rate of pattern vision recovery was also examined. Unlike strabismic humans and cats, the acuity deficit of monocularly deprived cats is not luminance dependent. This acuity is much worse at all luminance levels in the deprived eye. Likewise, initial testing of the deprived eye at low luminance levels does not facilitate recovery of pattern vision. Therefore, monocular lid fusion results in a visual deficit that differs from the effects of strabismus but resembles the effects of anisometropia in man.
Critical flicker frequency (CFF) was determined for both eyes of long-term monocularly deprived (... more Critical flicker frequency (CFF) was determined for both eyes of long-term monocularly deprived (MD) cats over a wide luminance range. Although MD cats could discriminate flicker before and after lid opening, CFF of the deprived eye (30 Hz) was much lower than CFF of the non-deprived eye (40 Hz) and the CFF of the non-deprived eye was lower than a normal cat&#39;s monocular CFF (58 Hz). The CFF deficit of the deprived eye became less pronounced at low luminance levels. The observation (and magnitude) of a CFF deficit for the deprived eye is compatible with the reports of a Y-cell loss in LGNd. The CFF deficit of the non-deprived eye has no obvious explanation.
Both humans and other mammals can more easily discriminate eight-sided figures from one another t... more Both humans and other mammals can more easily discriminate eight-sided figures from one another than they can figures having more or fewer sides. The hypothesis that this effect is based upon spatial contrast sensitivity was tested by varying viewing distance and hence spatial frequency content. Because viewing distance had little effect upon relative discrimination latency, it was concluded that the superior discriminability of eight-sided figures is not based upon spatial contrast sensitivity. The possibility that orientation tuning of visual cortex neurons may account for the effect is discussed.
Detectability was determined as a function of stimulus duration for the chromatic and achromatic ... more Detectability was determined as a function of stimulus duration for the chromatic and achromatic systems. For the achromatic system, detectability remains relatively constant for durations ranging from 43 to 1,000 msec, whereas for the chromatic system, detectability increases with duration up to about 500 msec. This suggests that the chromatic system has a more sustained output than the achromatic system. 65
The temporal appearances of spatially coincident increments that activate the chromatic or achrom... more The temporal appearances of spatially coincident increments that activate the chromatic or achromatic system are different. Whereas near-threshold stimuli that are detected by the chromatic system appear to come on gradually, increments detected by the achromatic system appear to flash or step on. This difference in perception is consistent with previous reports that show differencesin the temporal properties of the two systems.
We measured changes in the sensitivity of the human rod pathway by testing visual reaction times ... more We measured changes in the sensitivity of the human rod pathway by testing visual reaction times before and after light adaptation. We targeted a specific range of conditioning light intensities to see if a physiological adaptation recently discovered in mouse rods is observable at the perceptual level in humans. We also measured the noise spectrum of single mouse rods due to the importance of the signal-to-noise ratio in rod to rod bipolar cell signal transfer. Using the well-defined relationship between stimulus intensity and reaction time (Piéron's law), we measured the reaction times of eight human subjects (ages 24-66) to scotopic test flashes of a single intensity before and after the presentation of a 3-minute background. We also made recordings from single mouse rods and processed the cellular noise spectrum before and after similar conditioning exposures. Subject reaction times to a fixed-strength stimulus were fastest 5 seconds after conditioning background exposure (7...
CFF thresholds were determined for 3 normal and 3 binocularly deprived (BD) cats utilizing a simu... more CFF thresholds were determined for 3 normal and 3 binocularly deprived (BD) cats utilizing a simultaneous two-choice discrimination procedure. For all cats when adaptation level was steady-state for stimulus luminance. the CFF/luminana: function showed a rod-cone break which took the form of a mesopic null. When the cats were maintained in a light-adapted state CFF declined as a monotonic function of luminance. Under most luminance and adaptation levels the BD cats' CFF was slower than that of the normal cats. The BD cats' depressed CFF is interpreted as a result of their LGNd Y-cell loss.
Adult cats were trained to discriminate blue from green and gray. Although the cats could discrim... more Adult cats were trained to discriminate blue from green and gray. Although the cats could discriminate the intensity of stimuli whose areas ranged from 33 to 0.36 square centimeters they could not discriminate color when the stimulus was 0.36 square centimeter (less than 20 degrees visual angle). This influence of stimulus size may account for both positive and negative results of previous studies.
Our goal was to investigate the effect of displayed image magnification on perception of the size... more Our goal was to investigate the effect of displayed image magnification on perception of the size of hepatic lesions on abdominal computed tomography (CT) scans. Institutional review board approval and informed observer consent were obtained. Three experienced radiologists reviewed 90 CT image pairs in one session. Each image pair demonstrated a solitary, welldefined hypodense hepatic lesion measuring greater than 1 cm obtained at two points in time. The image pairs were presented three times in random order, once with the left image magnified, once with the right image magnified, and once with neither image magnified. The radiologists were asked to determine on which image the lesion was smaller or if there was no difference. The responses were analyzed statistically. The proportion of correct responses increased significantly as the difference in lesion size increased (p< 0.001). The percent of correct responses was higher when neither CT image was magnified. Magnification of one image decreased the accuracy of the readers' performance, especially at smaller differences, both of which were statistically significant (p<0.001). Thus, accuracy of detect-ing lesion size differences was degraded when the images were presented at differing magnification. This should be kept in mind when evaluating serial CT scans for growth or regression of tumors and other lesions.
The visual acuity of seven midland banded water snakes was measured by recording evoked responses... more The visual acuity of seven midland banded water snakes was measured by recording evoked responses from telencephalon to temporally modulated square wave grating patterns. Using conventional electrophysiological techniques and signal averaging, high contrast square wave gratings of diVerent spatial frequencies were presented. Acuity was estimated by extrapolating relative response amplitude/log 10 spatial frequency functions which yielded an average acuity of 4.25 cycles/degree. Refractive state was also estimated by recording evoked potentials to intermediate spatial frequencies with diVerent lenses in front of the eye. Polynomial Wts indicated that under the experimental conditions the snakes were around 6.4 diopters hyperopic suggesting a corrected acuity of 4.89 cycles/ degree. Reduction of grating luminance resulted in a reduction in evoked potential acuity measurements. These results indicate that the spatial resolution of midland banded water snakes is the equal of cat; about 20/120 in human clinical terms.
Four cats were raised with binocular eyelid suture and, after their eyes were opened, were traine... more Four cats were raised with binocular eyelid suture and, after their eyes were opened, were trained on a series of discrimination tasks. They performed a t normal rates on the brightness task but indicated some difficulty with the pattern tasks. They then received large, bilateral occipito-temporal cortex ablations. Postoperatively, this in no observable way affected their visually guided orienting behavior, but it did destroy their capacity to perform the preoperatively learned pattern tasks. Postoperative performance on the brightness task remained good. These data indicate that, in these deprived cats, there is little or no cortical development for visual orienting, but cortex is necessary for visual discrimination learning.
Critical flicker fusion was determined over a wide luminance range for six Siamese and four norma... more Critical flicker fusion was determined over a wide luminance range for six Siamese and four normal cats. The Siamese cats had the lowest CFF at all luminance levels when compared to normal and binocularly deprived (BD) cats. The Siamese cat's inferior temporal resolution is most likely due to their profound y-cell loss because (1) CFF is proportional to y-cell population across normal, BD, and Siamese cats; and (2) unlike BD cats, the visual cortex of Siamese cats is comparatively normal. The implication of this finding for the CFF of dark-reared cats and normal cats viewing a stimulus which does not stimulate y-cell is discussed.
Thirteen-lined ground squirrels (Citellus tridecemlineatus) were trained on three two-choice visu... more Thirteen-lined ground squirrels (Citellus tridecemlineatus) were trained on three two-choice visual discrimination problems: light/dark, color and stripe orientation. After posterior neocortical lesions in one or two stages, they were tested on all three discriminations. The results demonstrate that animals with large posterior neocortical lesions which produced retrograde changes throughout the dorsal lateral geniculate (LGNd) were capable of light/dark and wavelength discrimination. These animals were not able to discriminate stripe orientation. It is proposed that wavelength discrimination depends on extrageniculostriate mechanisms in posterior neodecorticates of this species.
We were able to train cats raised with sutured eyelids to perform simple brightness discriminatio... more We were able to train cats raised with sutured eyelids to perform simple brightness discriminations before their lids were parted. If, and only if, a small hole was present in a lid, could some of the cats also perform a grating orientation discrimination.
Visual acuity was determined for both eyes of long-term monocular-deprived cats over a wide lumin... more Visual acuity was determined for both eyes of long-term monocular-deprived cats over a wide luminance range. The influence of luminance on the rate of pattern vision recovery was also examined. Unlike strabismic humans and cats, the acuity deficit of monocularly deprived cats is not luminance dependent. This acuity is much worse at all luminance levels in the deprived eye. Likewise, initial testing of the deprived eye at low luminance levels does not facilitate recovery of pattern vision. Therefore, monocular lid fusion results in a visual deficit that differs from the effects of strabismus but resembles the effects of anisometropia in man.
Critical flicker frequency (CFF) was determined for both eyes of long-term monocularly deprived (... more Critical flicker frequency (CFF) was determined for both eyes of long-term monocularly deprived (MD) cats over a wide luminance range. Although MD cats could discriminate flicker before and after lid opening, CFF of the deprived eye (30 Hz) was much lower than CFF of the non-deprived eye (40 Hz) and the CFF of the non-deprived eye was lower than a normal cat&#39;s monocular CFF (58 Hz). The CFF deficit of the deprived eye became less pronounced at low luminance levels. The observation (and magnitude) of a CFF deficit for the deprived eye is compatible with the reports of a Y-cell loss in LGNd. The CFF deficit of the non-deprived eye has no obvious explanation.
Both humans and other mammals can more easily discriminate eight-sided figures from one another t... more Both humans and other mammals can more easily discriminate eight-sided figures from one another than they can figures having more or fewer sides. The hypothesis that this effect is based upon spatial contrast sensitivity was tested by varying viewing distance and hence spatial frequency content. Because viewing distance had little effect upon relative discrimination latency, it was concluded that the superior discriminability of eight-sided figures is not based upon spatial contrast sensitivity. The possibility that orientation tuning of visual cortex neurons may account for the effect is discussed.
Detectability was determined as a function of stimulus duration for the chromatic and achromatic ... more Detectability was determined as a function of stimulus duration for the chromatic and achromatic systems. For the achromatic system, detectability remains relatively constant for durations ranging from 43 to 1,000 msec, whereas for the chromatic system, detectability increases with duration up to about 500 msec. This suggests that the chromatic system has a more sustained output than the achromatic system. 65
The temporal appearances of spatially coincident increments that activate the chromatic or achrom... more The temporal appearances of spatially coincident increments that activate the chromatic or achromatic system are different. Whereas near-threshold stimuli that are detected by the chromatic system appear to come on gradually, increments detected by the achromatic system appear to flash or step on. This difference in perception is consistent with previous reports that show differencesin the temporal properties of the two systems.
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Papers by Michael Loop