Spatial frequency bandwidth used in the recognition of facial images

AbstractThe purpose of the study was to find out what spatial frequency information human observers use in the recognition of face images. Signal-to-noise ratio thresholds for the recognition of facial images were measured as a function of the centre spatial frequency of narrow-band additive spatial noise. The relative sensitivity of recognition to different spatial frequencies was derived from these results. The maximum sensitivity was found at 8–13 c/face width and the bandwidth was just under two octaves. Qualitatively similar results were obtained with stimuli in which Fourier phase was randomised within a narrow band of different centre spatial frequencies. This resulted in a considerable increase of energy threshold around 8 c/face width and less elsewhere. Further, contrast energy thresholds were measured as a function of the centre spatial frequency of band-pass filtered face images. As a function of object spatial frequency (c/face width), energy threshold first decreased and then increased. The lowest energy thresholds found around 10 c/face width were lower than the energy threshold for unfiltered images. This is what one would expect if face recognition is narrow-band, since band-pass filtered images of optimal centre spatial frequency do not contain unused contrast energy at low and high spatial frequencies. In conclusion, the results suggest that the recognition of facial images is tuned to a relatively narrow band (<2 octaves) of mid object spatial frequencies

Recognition of band-pass filtered hand-written numerals in foveal and peripheral vision

AbstractThe purpose of the present study was to find out what differences between foveal and peripheral pattern recognition remain unexplained by the inhomogeneities of retinal sampling and the optics of the eye. We measured contrast thresholds for pattern recognition at different eccentricities. The effects of retinal sampling were homogenised by using M-scaling of the stimuli, and the effects of the optics of the eye were by-passed either by using strong external noise (signal-to-noise ratio is not affected by optical attenuation) or by computing retinal image contrast by means of the optical modulation transfer function. The stimuli were hand-written numerals filtered to two-octave bands of various centre object spatial frequencies (c/object). The results were described as contrast thresholds and recognition efficiency. At all eccentricities, lowest contrast thresholds and highest recognition efficiencies were found at medium object spatial frequencies. At high object spatial frequencies the peripheral retinal contrast thresholds and recognition efficiencies were nearly as good as at the fovea, but at low object spatial frequencies most of the data showed superiority of the fovea to the periphery. Therefore, at high object spatial frequencies peripheral recognition performance could be explained relatively well by the retinal sampling gradient, or equivalently by the cortical magnification factor, together with the effects of the optics of the eye. Some eccentricity dependent deterioration of recognition at low object spatial frequencies remained unexplained

Neural modulation transfer function of the human visual system at various eccentricities

AbstractWe measured r.m.s. contrast sensitivity as a function of retinal illuminance at various spatial frequencies within 3–37 deg of eccentricity in the nasal visual field. In dim light contrast sensitivity increased in proportion to the square root of retinal illuminance obeying the De Vries-Rose law but in bright light contrast sensitivity was independent of luminance following Weber's law. Critical retinal illuminance (Ic) marking the transition between the laws was found to be independent of grating area but proportional to the spatial frequency squared at all eccentricities, in agreement with the Van Nes-Bouman law of foveal vision. In addition, the proportionality constant was found to be independent of eccentricity and similar to that of the fovea. According to our contrast detection model of human vision the modulation transfer function (PMTF) of the neural visual pathways squared is directly proportional to the critical retinal illuminance. On this basis our result means thatPMTF is similar, i.e. equal to spatial frequency across the visual field, thus attenuating low spatial frequencies relatively more than high spatial frequencies. Hence, up to the spatial cut-off frequency determined by the lowest neural sampling density of each retinal location the neural modulation transfer function is independent of visual location

Visually lossless compression of digital hologram sequences

Digital hologram sequences have great potential for the recording of 3D scenes of moving macroscopic objects as their numerical reconstruction can yield a range of perspective views of the scene. Digital holograms inherently have large information content and lossless coding of holographic data is rather inefficient due to the speckled nature of the interference fringes they contain. Lossy coding of still holograms and hologram sequences has shown promising results. By definition, lossy compression introduces errors in the reconstruction. In all of the previous studies, numerical metrics were used to measure the compression error and through it, the coding quality. Digital hologram reconstructions are highly speckled and the speckle pattern is very sensitive to data changes. Hence, numerical quality metrics can be misleading. For example, for low compression ratios, a numerically significant coding error can have visually negligible effects. Yet, in several cases, it is of high interest to know how much lossy compression can be achieved, while maintaining the reconstruction quality at visually lossless levels. Using an experimental threshold estimation method, the staircase algorithm, we determined the highest compression ratio that was not perceptible to human observers for objects compressed with Dirac and MPEG- 4 compression methods. This level of compression can be regarded as the point below which compression is perceptually lossless although physically the compression is lossy. It was found that up to 4 to 7.5 fold compression can be obtained with the above methods without any perceptible change in the appearance of video sequences

3D perception of numerical hologram reconstructions enhanced by motion and stereo

We investigated the question of how the perception of 3D information of digital holograms reconstructed numerically and presented on conventional displays depends on motion and stereoscopic presentation. Perceived depth in an adjustable random pattern stereogram was matched to the depth in holographic objects. The objects in holograms were a microscopic biological cell and a macroscopic coil. Stereoscopic presentation increased perceived depth substantially in comparison to non-stereoscopic presentation. When stereoscopic cues were weak or absent e.g. because of blur, motion increased perceived depth considerably. However, when stereoscopic cues were strong, the effect of motion was small. In conclusion, for the maximisation of perceived 3D information of holograms on conventional displays, it seems highly beneficial to use the combination of motion and stereoscopic presentation

Visual perception of digital holograms on autostereoscopic displays

In digital holography we often capture optically a 3D scene and reconstruct the perspectives numerically. The reconstructions are routinely in the form of a 2D image slice, an extended focus image, or a depth map from a single perspective. These are fundamentally 2D (or at most 2.5D) representations and for some scenes are not certain to give the human viewer a clear perception of the 3D features encoded in the hologram (occlusions are not overcome, for example). As an intermediate measure towards a full-field optoelectronic display device, we propose to digitally process the holograms to allow them to be displayed on conventional autostereoscopic displays

Using traditional glass plate holograms to study visual perception of future digital holographic displays

We study observers looking at a 3D scene captured in a traditional glass plate hologram using eye-tracking. We compare this with stereoscopic and 2D images. Our results can guide development of future digital holographic displays

An image-based visual strategy for working with color contrasts during design

acceptedVersio

“A Little Silly and Empty-Headed ” – Older Adults’ Understandings of Social Networking Sites

This study suggests reasons for the absence of a growing proportion of the population, the so-called baby boomers, from the otherwise highly popular social networking sites. We explore how people of this age group understand social networking sites and how these understandings fit certain aspects of their life. Designing social networking sites that match older adults ’ life would increase their possibilities of coping with the changes related to their age and of contributing to the information society. In a qualitative study involving use of an existing social networking site, and group and personal interviews, we found that understanding the internet as a dangerous place, and social networking sites as places of socially unacceptable behavior, hinders the use of these technologies. To include older adults, we propose arrangement of social events for getting familiarized with these services and offering of clear and simple privacy management on the sites. These actions have implications for users of all ages. Author Keywords Social networking sites, social media, aging society, older adults. ACM Classification Keywords H5.m. Information interfaces and presentation (e.g.