Why and how is soft copy reading possible in clinical practice? (1/1010)

The properties of the human visual system (HVS) relevant to the diagnostic process are described after a brief introduction on the general problems and advantages of using soft copy for primary radiology interpretations. At various spatial and temporal frequencies the contrast sensitivity defines the spatial resolution of the eye-brain system and the sensitivity to flicker. The adaptation to the displayed radiological scene and the ambient illumination determine the dynamic range for the operation of the HVS. Although image display devices are determined mainly by state-of-the-art technology, analysis of the HVS may suggest technical characteristics for electronic displays that will help to optimize the display to the operation of the HVS. These include display size, spatial resolution, contrast resolution, luminance range, and noise, from which further consequences for the technical components of a monitor follow. It is emphasized that routine monitor quality control must be available in clinical practice. These image quality measures must be simple enough to be applied as part of the daily routine. These test instructions might also serve as elements of technical acceptance and constancy tests.  (+info)

Orientation-tuned spatial filters for texture-defined form. (2/1010)

Detection threshold for an orientation-texture-defined (OTD) test grating was elevated after adapting to an OTD grating of high orientation contrast. Threshold elevation was greatest for a test grating parallel to the adapting grating, and fell to zero for a test grating perpendicular to the adapting grating. We conclude that the human visual system contains an orientation-tuned neural mechanism sensitive to OTD form, and propose a model for this mechanism. We further propose that orientation discrimination for OTD bars and gratings is determined by the relative activity of these filters for OTD form.  (+info)

Local velocity representation: evidence from motion adaptation. (3/1010)

Adaptation to a moving visual pattern induces shifts in the perceived motion of subsequently viewed moving patterns. Explanations of such effects are typically based on adaptation-induced sensitivity changes in spatio-temporal frequency tuned mechanisms (STFMs). An alternative hypothesis is that adaptation occurs in mechanisms that independently encode direction and speed (DSMs). Yet a third possibility is that adaptation occurs in mechanisms that encode 2D pattern velocity (VMs). We performed a series of psychophysical experiments to examine predictions made by each of the three hypotheses. The results indicate that: (1) adaptation-induced shifts are relatively independent of spatial pattern of both adapting and test stimuli; (2) the shift in perceived direction of motion of a plaid stimulus after adaptation to a grating indicates a shift in the motion of the plaid pattern, and not a shift in the motion of the plaid components; and (3) the 2D pattern of shift in perceived velocity radiates away from the adaptation velocity, and is inseparable in speed and direction of motion. Taken together, these results are most consistent with the VM adaptation hypothesis.  (+info)

The effect of spatial frequency adaptation on the latency of spatial contrast detection. (4/1010)

The effect of spatial frequency adaptation on detection response time was studied using 2-D Gabor functions as stimuli. On the basis of pilot studies, it was expected that reaction time to a given spatial frequency at a low contrast would increase following adaptation to that spatial frequency at a high contrast. Subjects were tested using 2-D Gabor functions that ranged in frequency from 25 to 24 cpd. Subjects' reaction times to the Gabor functions were measured prior to adaptation and after adaptation to a particular spatial frequency. The adapting spatial frequency was either 1, 2, 4, 8, 10, or 16 cpd. The test stimuli were 0.3, 0.5, or 0.7 log units above the unadapted threshold contrast. The subjects adapted to the high contrast test grating for 3 min (80% contrast) and reaction times were again measured in an adapt-test-readapt paradigm. The results showed the greatest increase in reaction time after adaptation when adapting and test spatial frequencies within an octave of the adapting spatial frequency also showed an increase in reaction time but to a lesser extent. Reaction times to gratings with spatial frequencies more distant from the adapting spatial frequency were not significantly affected by the adaptation. The results obtained resemble the tuning curves found for threshold data. Reaction times for stimuli at 0.5 and 0.7 log units above the unadapted threshold were affected less by adaptation than those at 0.3 log units above the unadapted threshold. These results were evaluated in terms of a shifting contrast gain mechanism which may account for both the spatial frequency specific effects of adaptation and the differences found for the different contrast test levels.  (+info)

S-cone signals to temporal OFF-channels: asymmetrical connections to postreceptoral chromatic mechanisms. (5/1010)

Psychophysical tests of S-cone contributions to temporal ON- and OFF-channels were conducted. Detection thresholds for S-cone modulation were measured with two kinds of test stimuli presented on a CRT: a rapid-on sawtooth test and a rapid-off sawtooth test, assumed to be detected differentially by temporal ON- and OFF-channels, respectively. S-cone related ON- and OFF-temporal responses were separated by adapting for 5 min to 1 Hz monochromatic (420, 440, 450, 540, or 650 nm in separate sessions) sawtooth flicker presented in Maxwellian view. Circular test stimuli, with a sawtooth temporal profile and a Gaussian spatial taper, were presented for 1 s in one of four quadrants 1.0 degree from a central fixation point. A four-alternative forced-choice method combined with a double-staircase procedure was used to determine ON- and OFF-thresholds in the same session. Following adaptation, the threshold elevation was greater if the polarity of the test stimulus was the same as the polarity of the sawtooth adaptation flicker, consistent with separate ON- and OFF-responses from S-cones. This asymmetrical pattern was obtained, however, only when the adaptation stimuli appeared blue with a little redness. When the adaptation flicker had a clear reddish hue component, the threshold elevation did not depend on the polarity of the sawtooth test stimuli. These results are consistent with a model in which OFF-signals originating from S cones are maintained by a postreceptoral mechanism signaling redness, but not by a postreceptoral chromatic mechanism signaling blueness.  (+info)

Dopamine mediates circadian rhythms of rod-cone dominance in the Japanese quail retina. (6/1010)

A circadian clock modulates the functional organization of the Japanese quail retina. Under conditions of constant darkness, rods dominate electroretinogram (ERG) b-wave responses at night, and cones dominate them during the day, yielding a circadian rhythm in retinal sensitivity and rod-cone dominance. The activity of tyrosine hydroxylase, the rate-limiting enzyme in dopamine synthesis, also exhibits a circadian rhythm in the retina with approximately threefold higher levels during the day than at night. The rhythm of tyrosine hydroxylase activity is opposite in phase to the circadian activity of tryptophan hydroxylase, the first enzyme in the melatonin biosynthetic pathway. We tested whether dopamine may be related to the physiological rhythms of the retina by examining the actions of pharmacological agents that effect dopamine receptors. We found that blocking dopamine D2 receptors in the retina during the day mimics the nighttime state by increasing the amplitude of the b-wave and shifting the retina to rod dominance. Conversely, activating D2 receptors at night mimics the daytime state by decreasing the amplitude of the b-wave and shifting the retina to cone dominance. A selective antagonist for D1 dopamine receptors has no effect on retinal sensitivity or rod-cone dominance. Reducing retinal dopamine partially abolishes rhythms in sensitivity and yields a rod-dominated retina regardless of the time of day. These results suggest that dopamine, under the control of a circadian oscillator, has a key role in modulating sensitivity and rod-cone dominance in the Japanese quail retina.  (+info)

Analysis of red/green color discrimination in subjects with a single X-linked photopigment gene. (7/1010)

Many subjects despite having only a single X-linked pigment gene (single-L/M-gene subjects) are able to make chromatic discriminations by Rayleigh matching, especially when large fields are used. We used a combination of psychophysics (Rayleigh match), electroretinograms (ERG), and molecular genetic techniques to rule out several possible explanations of this phenomenon. Use of rods for chromatic discrimination was unlikely since strong adapting fields were employed and the large-field match results were not consistent with rod participation. A putative mid- to long-wavelength photopigment that escapes detection by current molecular genetic analysis was ruled out by finding only a single L/M photopigment in flicker ERGs from 16 single-L/M-gene subjects. Large-field match results were not consistent with participation of S cones. Amino acid sequence polymorphisms in the S-pigment gene that might have shifted the S cone spectrum towards longer wavelengths were not found on sequencing. The mechanism of chromatic discrimination in the presence of a single photopigment therefore remains unknown. Further possible explanations such as variations in cone pigment density and retinal inhomogeneities are discussed.  (+info)

Time course of motion adaptation: motion-onset visual evoked potentials and subjective estimates. (8/1010)

The aim of this study was to quantitatively describe the dynamics of adaptation to visual motion with electrophysiological and psychophysical methods in man. We recorded visual evoked potentials (VEPs) to motion onset of random dot patterns from occipital and occipito-temporal electrodes during a succession of adaptation-recovery sequences. In these sequences the test stimulus was used to set the adaptation level: seven trials with 70% motion duty cycle (adaptation) followed by seven trials of 7% motion duty cycle (recovery). In a similar paradigm we determined the length of the perceptual motion after-effect to obtain a psychophysical measure of the time course of motion adaptation. Our results show a highly significant reduction of the N2 amplitude in the maximally compared to the minimally adapted condition (P < 0.001). Electrophysiological and psychophysical results both indicate that adaptation to visual motion is faster than recovery: The data were fit with an exponential model yielding adaptation and recovery time constants, respectively, of 2.5 and 10.2 s for the N2 amplitude (occipito temporal derivation) and of 7.7 and 16.7 s for the perceptual motion after-effect. Implications for the design of motion stimuli are discussed, e.g. a motion stimulus moving 10% of the time may lead to about 30% motion adaptation.  (+info)