PLT OF A PA44 FAILED TO FOLLOW THE DIRECT TRACK TO ARPT INSTRUCTED BY APCH CTLR CAUSING THE CTLR TO ALERT THE RPTR AND PROVIDE A HDG THAT WOULD AVOID CONFLICTING TFC.

Narrative

I WAS RETURNING FROM A ROUND TRIP TO NORTH CAROLINA WITH ALMOST 4 HRS OF FLT TIME BEHIND ME. AS I APCHED THE BALTIMORE AREA; I WAS DSNDING THROUGH A CLOUD LAYER AND BROKE OUT AROUND 4500 FT. DURING MY DSCNT; I HAD PROGRAMMED THE GPS FOR THE 'ILS 33' APCH INTO MARTIN STATE (MTN) AND WAS REVIEWING THE APCH PLATE. THE BALTIMORE CTLR ADVISED ME TO 'PROCEED DIRECT MARTIN STATE FOR THE VISUAL RWY 33 APCH;' FROM MY POS JUST W OF EASTON. HIS CLRNC TO ME WAS FINE AND I EVEN ACKNOWLEDGED IT; BUT IN MY MIND I WAS STILL THINKING 'ILS RWY 33' APCH. FROM MY POS; IN ORDER TO PROCEED DIRECT TO MTN; I SHOULD HAVE FLOWN A COURSE OF ABOUT 015 DEGS; BUT SINCE I WAS ACTUALLY STILL THINKING THE ILS RWY 33; I BEGAN TO FLY TOWARD THE IAF (SWANN) WHICH WAS MORE ON A HDG OF 025 DEGS. THIS PUT ME PROBABLY 4-5 MI E OF THE MORE DIRECT TRACK THAT THE CTLR WAS EXPECTING ME TO TAKE. WHEN THE CTLR SAW ME 'OFF COURSE' BY SEVERAL MI; HE ADVISED ME THAT MY CLRNC WAS FOR 'DIRECT MARTIN STATE FOR THE VISUAL RWY 33.' HE ADVISED THAT I COULD HAVE CAUSED A TFC CONFLICT HAD I CONTINUED. I THEN TURNED AS PER HIS INSTRUCTIONS AND SOON CALLED THE ARPT IN SIGHT WITH A NON EVENTFUL APCH (VISUAL) AND LNDG. I BELIEVE THE PROB WAS THAT ALTHOUGH APCH CTL HAD ISSUED A FAMILIAR 'VISUAL APCH' CLRNC; MY MIND WAS 'PREPROGRAMMED' FOR A DIFFERENT APCH TO THE SAME RWY BECAUSE OF MY GPS PROGRAMMING AND APCH PLATE REVIEW ONLY MOMENTS BEFORE. I WILL BE VERY CAREFUL TO LISTEN AND COMPREHEND FUTURE CLRNCS OF ALL KINDS.