Measuring FMD in the brachial artery: how important is QRS gating?

Recommendations for the measurement of brachial flow-mediated dilation (FMD) typically suggest images be obtained at identical times in the cardiac cycle, usually end diastole (QRS complex onset). This recommendation presumes that inter-individual differences in arterial compliance are minimized. However, published evidence is conflicting. Furthermore, ECG gating is not available on many ultrasound systems; it requires an expensive software upgrade or increased image processing time. We tested whether analysis of images acquired with QRS gating or with the more simplified method of image averaging would yield similar results. We analyzed FMD and nitroglycerin-mediated dilation (NMD) in 29 adults with type 2 diabetes mellitus and in 31 older adults and 12 young adults without diabetes, yielding a range of brachial artery distensibility. FMD and NMD were measured using recommended QRS-gated brachial artery diameter measurements and, alternatively, the average brachial diameters over the entire R-R interval. We found strong agreement between both methods for FMD and NMD (intraclass correlation coefficients = 0.88-0.99). Measuring FMD and NMD using average diameter measurements significantly reduced post-image-processing time (658.9 ± 71.6 vs. 1,024.1 ± 167.6 s for QRS-gated analysis, P < 0.001). FMD and NMD measurements based on average diameter measurements can be performed without reducing accuracy. This finding may allow for simplification of FMD measurement and aid in the development of FMD as a potentially useful clinical tool.

Investigating retrospective influences on induction in rats' responding for 1% sucrose when food-pellet reinforcement is upcoming.

When rats lever press for 1% sucrose reinforcement in the first half of a 50-min session, response rates are higher when food-pellet reinforcement will be available in the second half than when 1% sucrose will be available. Results of past research have suggested that, under some conditions, this induction effect is prospective in nature (i.e., controlled by the conditions of reinforcement in the present session). However, that research did not rule out the possibility that, under other conditions, retrospective factors (i.e., the conditions of reinforcement in the previous session[s]) could contribute. In the present study, rats responded in two types of session, one in which 1% sucrose reinforcement was available in both halves of the session and one in which 1% sucrose and food-pellet reinforcement were available in the first and second halves, respectively. Which type of session was in effect unsignaled and session type alternated every session (Experiment 1), every second session (Experiment 2), or after at least 20 consecutive sessions of one type (Experiment 3). Across experiments, the results indicated that it takes several sessions of one type for observable retrospective effects to occur, but those effects are short lived. These results allow the authors to identify the mechanisms that must underlie induction. The authors also discuss induction as an animal model of anticipation.

Is induction produced by upcoming food-pellet reinforcement the outcome of an increase in overall activity or in operant responding?

Researchers have demonstrated that rats reliably increase their rates of pressing a lever for 1% liquid-sucrose reinforcement if they will soon have the opportunity to press a lever for food-pellet reinforcement. In the present experiments, the authors investigated if this increase in response rates occurred because the upcoming food pellets produced an increase in all behaviors (i.e., general arousal) or an increase in only the specific operant response (i.e., lever pressing). The results of Experiments 1 and 2 showed that the appearance of induction in rats' lever pressing for 1% sucrose reinforcement when food-pellet reinforcement was upcoming did not coincide with increases in the frequency of running in a wheel or making a nonreinforced nose-poke response. On the other hand, in Experiment 3, the authors found the appearance of induction coincided with increase nonreinforced lever presses on an adjacent lever. These results shed doubt on the idea that induction is a result of a general increase in all activity, and suggest instead that the increase in responding that occurs during induction is limited to the operant response.

Duration, response, and location: the influence of upcoming 32% sucrose on rats' licking or lever pressing for 1% liquid sucrose.

The present study investigated whether rats' rates of licking or pressing a lever for 1% liquid sucrose delivered by a continuous reinforcement schedule would decrease (contrast) or increase (induction) when the upcoming period would allow access to 32% sucrose and whether such changes would be influenced by how long each substance was available. In Experiment 1, different groups of rats licked a spout or pressed a lever for 1% sucrose in the first half of the session and, in different conditions, for 1% or 32% sucrose in the second half. Across conditions, halves of the session were 3, 6, 12, or 24 min long. Upcoming 32% sucrose significantly decreased rates of licking at each duration whereas it increased rates of lever pressing except when access duration was 3 min. Experiment 2 replicated Experiment 1 with the exception that rats that licked did so from the same spout in both halves of the session and rats that pressed a lever collected the sucrose reinforcers in the different halves at different locations. In these procedures, upcoming 32% sucrose significantly increased rates of licking. Significant, but small, increases in rates of lever pressing were still observed. The present results suggest that continuous reinforcement or duration of access to sucrose are not primary determinants of whether contrast or induction is observed. Rather, they suggest that the type of behavior (licking versus pressing a lever) and the location at which the substances are collected and consumed play a large role in which effect occurs.

Altering "motivational" variables alters induction produced by upcoming food-pellet reinforcement.

Previous research has demonstrated that rats will increase their rates of lever pressing for sucrose rewards in the first half of an experimental session when food pellets, rather than the same sucrose, continually serve as the reward in the second half of the session. This effect has been coined induction, and the present study investigated whether it could be altered by altering "motivational" variables. Experiment 1 manipulated subjects' motivation by altering, across conditions, their level of food deprivation. Predictably, the size of induction varied directly with level of deprivation. Experiments 2 and 3 manipulated subjects' motivation by feeding them food pellets and sucrose, respectively, prior to their responding in the experimental session. These pre-session feedings decreased the size of the observed induction in both experiments. The results from the present study indicate that the size of induction is correlated with subjects' motivation to respond for the available reinforcers. They are also consistent with the idea that operant processes underlie the effect. The notion that induction might encompass the concept of "anticipation" is also discussed.