Naveen K Challa
Staar Surgicals

Allied Health Science



  • Naveen K Challa
    [Show abstract] [Hide abstract]
    ABSTRACT: he work in this book is concerned with examining the retinal contributions to human trichromatic colour vision. Chromatic processing at the retinal level was examined using the electroretinograms (ERGs) for which cone isolating stimuli were used to assess the nature of L and M cone inputs to cone-opponent mechanisms. From the experiments, It has been shown that the low (12Hz) and high (30Hz) temporal frequency flickering stimuli can isolate the chromatic and luminance processing mechanisms in the retina. For low temporal frequency ERGs, the L:M ratio was close to unity and L/M phase difference was close to 180°.For high temporal frequency ERGs, the L:M ratio was more than unity and L/M phase difference was close to 90°. In addition to this, the variation in L:M ratio across the retinal eccentricity was also examined. These results suggest, for the chromatic processing, L:M ratio is close to unity independent of retinal eccentricity and individuals. For the luminance processing, L:M ratio is more than unity and depends upon the region of the retina being stimulated. These findings indicate the maintenance of cone selective input for the chromatic processing across the human retina.
    No preview · Book · Jul 2014
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Electroretinograms (ERGs) elicited by transient, square-wave L- and M-cone isolating stimuli were recorded from human trichromatic (n=19) and dichromatic (n=4) observers. The stimuli were generated on a four primary LED stimulator and were equated in terms of cone modulation (cone contrast=0.11) and retinal illuminance (12,000 trolands). L- and M-cone isolated ERGs had waveforms similar to those observed for luminance responses. However, M-cone ERGs exhibited a phase reversal in their responses to onset and offset stimuli relative to the L-cone responses. This on-off response reversal was observed in trichromats but not dichromats. Simultaneous counterphase and inphase combinations of L- and M-cone isolating stimuli generated responses that reflected chromatic and luminance processing, respectively. We conclude that L- and M-cone specific ERGs provide a measure of how photoreceptors contribute to postreceptoral mechanisms.
    Full-text · Article · Apr 2014 · Journal of the Optical Society of America A
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We recorded L- and M-cone isolating ERGs from human subjects using a silent substitution technique at temporal rates of 12 and 30 Hz. These frequencies isolate the activity of cone-opponent and non-opponent post-receptoral mechanisms, respectively. ERGs were obtained using a sequence of stimuli with different spatial configurations comprising; (1) circular stimuli of different sizes which increased in 10° steps up to 70°diameter, or (2) annular stimuli with a 70° outer diameter but with different sized central ablations from 10° up to 60°. L- and M-cone isolating ERGs were obtained from five colour normal subjects using a DTL fibre electrode. Fourier analysis of the ERGs was performed and we measured the amplitude of the first harmonic of the response. For 12 Hz ERGs the L:M cone response amplitude ratio (L:M(ERG)) was close to unity and remained stable irrespective of the spatial configuration of the stimulus. The maintenance of this balanced ratio points to the existence of cone selective input across the human retina for the L-M cone opponent mechanism. For 30 Hz the L:M(ERG) ratio was greater than unity but varied depending upon which region of the retina was being stimulated. This variation we consider to be a consequence of the global response properties of M-cone ERGs rather than representing a real variation in L:M cone ratios across the retina.
    Full-text · Article · Sep 2010 · Ophthalmic and Physiological Optics
  • [Show abstract] [Hide abstract]
    ABSTRACT: Purpose: : To characterize the retinal eccentricity dependency of L- and M-cone inputs to the cone opponent and non-opponent post-receptoral mechanisms in human observers with flicker ERGs. Methods: : ERGs were obtained from a series of flickering stimuli (either 30Hz or 12Hz previously found to reflect activity of the non-opponent and the L-/M-cone opponent channels respectively, Kremers & Link, J Vis 8, 1-14) using the following spatial configurations: (1) Circular stimuli of different angular subtense which increased in 10° steps up to 70° diameter. (2) Annuli with 70° outer diameter and gradually ablated from the centre in 10° steps. Responses to L- and M-cone isolating stimuli with equal cone contrasts were obtained from colour normal subjects using a DTL electrode. The amplitudes and phases of the first harmonic components were obtained. Results: : The ratio of the L- and M-cone response amplitudes was found to be at or close to unity at 12Hz for central as well as peripheral stimuli and the L- and M-cone phase differences were close to 180o compared to the smaller phase differences measured for the 30Hz responses. At 30Hz the L-/M-cone ratio was found to vary from 4:1 to 10:1, for different observers. Those observers with small M-cone driven responses displayed larger L-/M- ratios with a large stimulus size compared with the small central and the far peripheral annular stimuli. In these subjects, the phases of the M-cone driven responses differed strongly for the central and peripheral stimuli. Conclusions: : The L-/M-cone-opponent post-receptoral mechanism seems to maintain its cone selective input in the peripheral human retina. In subjects with particularly small numbers of M-cones, the M-cone driven response in the non-opponent channel may have strongly different phases in central and peripheral retina which may cancel each other out when stimulated simultaneously leading to particularly large L-/M-ratios in when using large stimuli.
    No preview · Article · Apr 2010 · Investigative ophthalmology & visual science

  • No preview · Article · Dec 2009 · Journal of Vision

8 Following View all

45 Followers View all