Timing of visual stimuli in V1 and V5/MT: fMRI and TMS

Time perception is used in our day-to-day activities. While we understand quite well how our brain processes vision, touch or taste, brain mechanisms subserving time perception remain largely understudied. In this study, we extended an experiment of previous master thesis run by Tatiana Kenel-Pierre. We focused on time perception in the range of milliseconds. Previous studies have demonstrated the involvement of visual areas V1 and V5/MT in the encoding of temporal information of visual stimuli. Based on these previous findings the aim of the present study was to understand if temporal information was encoded in V1 and extrastriate area V5/MT in different spatial frames i.e., head- centered versus eye-centered. To this purpose we asked eleven healthy volunteers to perform a temporal discrimination task of visual stimuli. Stimuli were presented at 4 different spatial positions (i.e., different combinations of retinotopic and spatiotopic position). While participants were engaged in this task we interfered with the activity of the right dorsal V1 and the right V5/MT with transcranial magnetic stimulation (TMS). Our preliminary results showed that TMS over both V1 and V5/MT impaired temporal discrimination of visual stimuli presented at specific spatial coordinates. But whereas TMS over V1 impaired temporal discrimination of stimuli presented in the lower left quadrant, TMS over V5/MT affected temporal discrimination of stimuli presented at the top left quadrant. Although it is always difficult to draw conclusions from preliminary results, we could tentatively say that our data seem to suggest that both V1 and V5/MT encode visual temporal information in specific spatial frames.

Eight controversies

Review of the book: Eight preposterous propositions by Robert Ehrlich, Princeton University press, Princeton USA.

Another strand to the DNA story

Review of the book: The third man of the double Helix by Maurice Wilkins. 10.1038/sj.embor.7400062

Explaining explanations

Review of the book: explanations: Styles of Explanation in Science by John Cornwell (ed) Oxford University Press, Oxford, UK

Controlled contrast transcranial Doppler and arterial blood gas analysis to quantify shunt through patent foramen ovale.

BACKGROUND AND PURPOSE: A right-to-left shunt can be identified by contrast transcranial Doppler ultrasonography (c-TCD) at rest and/or after a Valsalva maneuver (VM) or by arterial blood gas (ABG) measurement. We assessed the influence of controlled strain pressures and durations during VM on the right-to-left passage of microbubbles, on which depends the shunt classification by c-TCD, and correlated it with the right-to-left shunt evaluation by ABG measurements in stroke patients with patent foramen ovale (PFO). METHODS: We evaluated 40 stroke patients with transesophageal echocardiography-documented PFO. The microbubbles were recorded with TCD at rest and after 4 different VM conditions with controlled duration and target strain pressures (duration in seconds and pressure in cm H2O, respectively): V5-20, V10-20, V5-40, and V10-40. The ABG analysis was performed after pure oxygen breathing in 34 patients, and the shunt was calculated as percentage of cardiac output. RESULTS: Among all VM conditions, V5-40 and V10-40 yielded the greatest median number of microbubbles (84 and 95, respectively; P<0.01). A significantly larger number of microbubbles were detected in V5-40 than in V5-20 (P<0.001) and in V10-40 than in V10-20 (P<0.01). ABG was not sensitive enough to detect a shunt in 31 patients. CONCLUSIONS: The increase of VM expiratory pressure magnifies the number of microbubbles irrespective of the strain duration. Because the right-to-left shunt classification in PFO is based on the number of microbubbles, a controlled VM pressure is advised for a reproducible shunt assessment. The ABG measurement is not sensitive enough for shunt assessment in stroke patients with PFO.

Time processing in visual cortices: How the visual brain encodes and keeps track of time

Time is embedded in any sensory experience: the movements of a dance, the rhythm of a piece of music, the words of a speaker are all examples of temporally structured sensory events. In humans, if and how visual cortices perform temporal processing remains unclear. Here we show that both primary visual cortex (V1) and extrastriate area V5/MT are causally involved in encoding and keeping time in memory and that this involvement is independent from low-level visual processing. Most importantly we demonstrate that V1 and V5/MT are functionally linked and temporally synchronized during time encoding whereas they are functionally independent and operate serially (V1 followed by V5/MT) while maintaining temporal information in working memory. These data challenge the traditional view of V1 and V5/MT as visuo-spatial features detectors and highlight the functional contribution and the temporal dynamics of these brain regions in the processing of time in millisecond range. The present project resulted in the paper entitled: 'How the visual brain encodes and keeps track of time' by Paolo Salvioni, Lysiann Kalmbach, Micah Murray and Domenica Bueti that is now submitted for publication to the Journal of Neuroscience.

Performance of classic electrocardiographic criteria for left ventricular hypertrophy in an African population.

ECG criteria for left ventricular hypertrophy (LVH) have been almost exclusively elaborated and calibrated in white populations. Because several interethnic differences in ECG characteristics have been found, the applicability of these criteria to African individuals remains to be demonstrated. We therefore investigated the performance of classic ECG criteria for LVH detection in an African population. Digitized 12-lead ECG tracings were obtained from 334 African individuals randomly selected from the general population of the Republic of Seychelles (Indian Ocean). Left ventricular mass was calculated with M-mode echocardiography and indexed to body height. LVH was defined by taking the 95th percentile of body height-indexed LVM values in a reference subgroup. In the entire study sample, 16 men and 15 women (prevalence 9.3%) were finally declared to have LVH, of whom 9 were of the reference subgroup. Sensitivity, specificity, accuracy, and positive and negative predictive values for LVH were calculated for 9 classic ECG criteria, and receiver operating characteristic curves were computed. We also generated a new composite time-voltage criterion with stepwise multiple linear regression: weighted time-voltage criterion=(0.2366R(aVL)+0.0551R(V5)+0.0785S(V3)+ 0.2993T(V1))xQRS duration. The Sokolow-Lyon criterion reached the highest sensitivity (61%) and the R(aVL) voltage criterion reached the highest specificity (97%) when evaluated at their traditional partition value. However, at a fixed specificity of 95%, the sensitivity of these 10 criteria ranged from 16% to 32%. Best accuracy was obtained with the R(aVL) voltage criterion and the new composite time-voltage criterion (89% for both). Positive and negative predictive values varied considerably depending on the concomitant presence of 3 clinical risk factors for LVH (hypertension, age >/=50 years, overweight). Median positive and negative predictive values of the 10 ECG criteria were 15% and 95%, respectively, for subjects with none or 1 of these risk factors compared with 63% and 76% for subjects with all of them. In conclusion, the performance of classic ECG criteria for LVH detection was largely disparate and appeared to be lower in this population of East African origin than in white subjects. A newly generated composite time-voltage criterion might provide improved performance. The predictive value of ECG criteria for LVH was considerably enhanced with the integration of information on concomitant clinical risk factors for LVH.

How the visual brain encodes and keeps track of time.

Time is embedded in any sensory experience: the movements of a dance, the rhythm of a piece of music, the words of a speaker are all examples of temporally structured sensory events. In humans, if and how visual cortices perform temporal processing remains unclear. Here we show that both primary visual cortex (V1) and extrastriate area V5/MT are causally involved in encoding and keeping time in memory and that this involvement is independent from low-level visual processing. Most importantly we demonstrate that V1 and V5/MT come into play simultaneously and seem to be functionally linked during interval encoding, whereas they operate serially (V1 followed by V5/MT) and seem to be independent while maintaining temporal information in working memory. These data help to refine our knowledge of the functional properties of human visual cortex, highlighting the contribution and the temporal dynamics of V1 and V5/MT in the processing of the temporal aspects of visual information.

Nonrandom Distribution of Cryptic Repeating Triplets of Purines and Pyrimidines (RNY)(n) in gp120 of HIV Type1.

Abstract We have analyzed purine (R) and pyrimidine (Y) codon patterns in variable and constant regions of HIV-1 gp120 in seven patients infected with different HIV-1 subtypes and naive to antiretroviral therapy. We have calculated the relative frequency of each in-frame codon RNY, YNR, RNR, and YNY (N=any nucleotide) in variable and constant regions of gp120, in the sequence within indels and at indels' flanking sites. Our data show that hypervariable regions V1, V2, V4, and V5 are characterized by the presence of long stretches of RNY codons constituting the majority of the sequence portion within insertions/deletions. In full-length gp120 and within inserted/deleted fragments the number of AVT (V=A, C, G) codons did not exceed 50% of the total RNY codons. RNY strings in variable regions spanned up to 21 codons and were always in frame. In contrast, RNY strings in constant regions were mostly out of frame and their length was limited to five codons. The frequency of the codon RNY was found to be significantly higher in variable regions (p<0.0001; t-test), within indels, and at indels' flanking sites (p<0.0001; χ(2) test). Analysis of the distribution of RNY strings equal to or longer than five codons in the full genome of HXB2 also shows that these sequences are mostly out of frame, unless they contain a potential N-glycosylation site or an asparagine. These data suggest that cryptic repeats of RNY may play a role in the genesis of multiple base insertions and deletions in hypervariable regions of gp120.